Add detailed README for Knowledge Component

Introduced a comprehensive README file for the Knowledge Component. The new documentation provides an overview of the component, including its database schema, operations available to peers, and ER diagram. It also explains the algorithm behind the community's operation and outlines changes made to the UI related to this component. This update aims to improve understanding of how the Knowledge Component functions within the larger system.

src/tribler/core/components/knowledge/README.md

@@ -0,0 +1,205 @@
+Below you can find a brief description of the Knowledge Component. For more
+information please check the source code and https://github.com/Tribler/tribler/issues/6214
+
+# Database
+
+The full schema is available at
+https://github.com/Tribler/tribler/blob/main/src/tribler/core/components/database/db/layers/knowledge_data_access_layer.py
+
+It describes `Statements` that are linked to a `Peer` (through a digital signature) who created that statement. To
+anonymize the peer, not their main public key is used, but a secondary key as described
+
+```python
+        # secondary key:
+        secondary_private_key_path = config.state_dir / config.trustchain.secondary_key_filename
+        self.secondary_key = self.load_or_create(secondary_private_key_path)
+```
+https://github.com/Tribler/tribler/blame/26b0be8c4010a6546f9204bb2f0026597dea53a7/src/tribler/core/components/key/key_component.py#L24-L26
+
+In the `KnowledgeGraph`, a `Statement` is an edge with the following (simplified) structure:
+
+```python
+@dataclass
+class SimpleStatement:
+    subject_type: ResourceType
+    subject: str
+    predicate: ResourceType
+    object: str
+```
+https://github.com/Tribler/tribler/blob/76de5620ed7f1e991e9d1adc4cb905ae8a5db8b3/src/tribler/core/components/database/db/layers/knowledge_data_access_layer.py#L60-L65
+
+Where `ResourceType` is
+
+```python
+class ResourceType(IntEnum):
+    """ Description of available resources within the Knowledge Graph.
+    These types are also using as a predicate for the statements.
+
+    Based on https://en.wikipedia.org/wiki/Dublin_Core
+    """
+    CONTRIBUTOR = 1
+    COVERAGE = 2
+    CREATOR = 3
+    DATE = 4
+    DESCRIPTION = 5
+    FORMAT = 6
+    IDENTIFIER = 7
+    LANGUAGE = 8
+    PUBLISHER = 9
+    RELATION = 10
+    RIGHTS = 11
+    SOURCE = 12
+    SUBJECT = 13
+    TITLE = 14
+    TYPE = 15
+
+    # this is a section for extra types
+    TAG = 101
+    TORRENT = 102
+    CONTENT_ITEM = 103
+```
+https://github.com/Tribler/tribler/blob/26b0be8c4010a6546f9204bb2f0026597dea53a7/src/tribler/core/components/database/db/layers/knowledge_data_access_layer.py#L32-L57
+
+Statement examples:
+
+```python
+SimpleStatement(subject_type=ResourceType.TORRENT, subject='infohash1', predicate=ResourceType.TAG, object='tag1')
+SimpleStatement(subject_type=ResourceType.TORRENT, subject='infohash2', predicate=ResourceType.TAG, object='tag2')
+SimpleStatement(subject_type=ResourceType.TORRENT, subject='infohash3', predicate=ResourceType.CONTENT_ITEM,
+                object='content item')
+```
+
+Due to the inherent lack of trust in peers, we cannot simply replace an existing statement with a newly received one.
+Instead, we store all `statement-peer` pairs. This approach allows for the potential to downvote or upvote all
+statements associated with a particular peer, depending on whether that peer becomes trusted or not. Currently, we lack
+a reliable trust function, and this design was implemented with the anticipation that such a function would be developed
+in the future. This assumption has proven to be correct, as the design is compatible with the `MeritRank`. Furthermore,
+it appears that all necessary data is already present within the `KnowledgeGraph`, making it well-suited for future
+integration with trust evaluation mechanisms.
+
+There are two operations available for peers:
+
+```python
+class Operation(IntEnum):
+    """ Available types of statement operations."""
+    ADD = 1  # +1 operation
+    REMOVE = 2  # -1 operation
+```
+https://github.com/Tribler/tribler/blob/26b0be8c4010a6546f9204bb2f0026597dea53a7/src/tribler/core/components/database/db/layers/knowledge_data_access_layer.py#L26-L29
+
+All operations are recorded in the database, allowing for the calculation of the final score of a specific operation
+based on the cumulative actions taken by all peers. This approach enables a comprehensive assessment of each operation's
+overall impact within the network.
+
+We use clocks (lamport-like) for operations based on triplets {public_key, operation, infohash}
+
+Currently, a simplistic approach is employed, which involves merely summing all the 'add' operations (+1) and
+subtracting the 'remove' operations (-1) across all peers. This method is intended to be replaced by a more
+sophisticated mechanism, the `MeritRank` merit function.
+
+
+https://github.com/Tribler/tribler/blob/26b0be8c4010a6546f9204bb2f0026597dea53a7/src/tribler/core/components/database/db/layers/knowledge_data_access_layer.py#L98-L100
+
+### ER diagram
+
+```mermaid
+erDiagram
+    Peer {
+        int id PK "auto=True"
+        bytes public_key "unique=True"
+        datetime added_at "Optional, default=utcnow()"
+    }
+
+    Statement {
+        int id PK "auto=True"
+        int subject_id FK
+        int object_id FK
+        int added_count "default=0"
+        int removed_count "default=0"
+        int local_operation "Optional"
+    }
+
+    Resource {
+        int id PK "auto=True"
+        string name
+        int type "ResourceType enum"
+    }
+
+    StatementOp {
+        int id PK "auto=True"
+        int statement_id FK
+        int peer_id FK
+        int operation
+        int clock
+        bytes signature
+        datetime updated_at "default=utcnow()"
+        bool auto_generated "default=False"
+    }
+
+    Misc {
+        string name PK
+        string value "Optional"
+    }
+
+
+    Statement }|--|| Resource : "subject_id"
+    Statement }|--|| Resource : "object_id"
+    StatementOp }|--|| Statement : "statement_id"
+    StatementOp }|--|| Peer : "peer_id"
+```
+
+# Community
+
+https://github.com/Tribler/tribler/blob/main/src/tribler/core/components/knowledge/community/knowledge_community.py
+
+The algorithm of the community's operation:
+
+1. Every 5 seconds, we request 10 `StatementOperations` from a random peer.
+https://github.com/Tribler/tribler/blob/44e2235e0b3bcdc12ae2fcd874bc058474973e5b/src/tribler/core/components/knowledge/community/knowledge_payload.py#L8-L18
+```python
+@dataclass
+class StatementOperation:
+    """Do not change the format of the StatementOperation, because this will result in an invalid signature.
+    """
+    subject_type: int  # ResourceType enum
+    subject: str
+    predicate: int  # ResourceType enum
+    object: str
+    operation: int  # Operation enum
+    clock: int  # This is the lamport-like clock that unique for each quadruple {public_key, subject, predicate, object}
+    creator_public_key: type_from_format('74s')
+```
+2. Upon receiving a response, we verify the signatures of the operations and their validity.
+```python
+    def verify_signature(self, packed_message: bytes, key: Key, signature: bytes, operation: StatementOperation):
+        if not self.crypto.is_valid_signature(key, packed_message, signature):
+            raise InvalidSignature(f'Invalid signature for {operation}')
+```   
+https://github.com/Tribler/tribler/blob/44e2235e0b3bcdc12ae2fcd874bc058474973e5b/src/tribler/core/components/knowledge/community/knowledge_community.py#L126-L128
+
+```python
+    def validate_operation(operation: StatementOperation):
+        validate_resource(operation.subject)
+        validate_resource(operation.object)
+        validate_operation(operation.operation)
+        validate_resource_type(operation.subject_type)
+        validate_resource_type(operation.predicate)
+```
+https://github.com/Tribler/tribler/blob/44e2235e0b3bcdc12ae2fcd874bc058474973e5b/src/tribler/core/components/knowledge/community/knowledge_community.py#L119-L124
+
+3. When `StatementOperations` are requested from us, we select N random operations (the number of operations is
+   specified in the request) and return them.
+
+Design decisions behind the Community:
+- Knowledge operations gossiped transitively across the network
+- Pull-based gossip is used
+- For knowledge' integrity check ipv8 signatures are used
+
+# UI
+
+Two changes have been made to the UI:
+1. Elements for displaying tags (this feature is hided at the current moment).
+![image](https://github.com/Tribler/tribler/assets/13798583/2c3e6b98-71fc-42be-ab65-25bcd9012215)
+
+2. A dialog for editing metadata:
+![image](https://github.com/Tribler/tribler/assets/13798583/14c309ef-8f30-4710-a02e-0338fda60508)