adding flatten-graph token filter docs opensearch-project#8060

Signed-off-by: Anton Rubin <[email protected]>

_analyzers/token-filters/flatten-graph.md

@@ -0,0 +1,106 @@
+---
+layout: default
+title: Flatten graph
+parent: Token filters
+nav_order: 150
+---
+
+# Flatten graph token filter
+
+The `flatten_graph` token filter is used to handle complex token relationships that occur when multiple tokens are generated at the same position in a "graph" structure. This is particularly useful for synonym processing or other token alterations where tokens need to be restructured in a simplified format, making it compatible with operations that can’t handle more complex token structures.
+
+## Example
+
+The following example request creates a new index named `test_index` and configures an analyzer with a `flatten_graph` filter:
+
+```json
+PUT /test_index
+{
+  "settings": {
+    "analysis": {
+      "analyzer": {
+        "my_index_analyzer": {
+          "type": "custom",
+          "tokenizer": "standard",
+          "filter": [
+            "my_custom_filter",
+            "flatten_graph"
+          ]
+        }
+      },
+      "filter": {
+        "my_custom_filter": {
+          "type": "word_delimiter_graph",
+          "catenate_all": true
+        }
+      }
+    }
+  }
+}
+```
+{% include copy-curl.html %}
+
+## Generated tokens
+
+Use the following request to examine the tokens generated using the analyzer:
+
+```json
+POST /test_index/_analyze
+{
+  "analyzer": "my_index_analyzer",
+  "text": "OpenSearch helped many employers"
+}
+```
+{% include copy-curl.html %}
+
+The response contains the generated tokens:
+
+```json
+{
+  "tokens": [
+    {
+      "token": "OpenSearch",
+      "start_offset": 0,
+      "end_offset": 10,
+      "type": "<ALPHANUM>",
+      "position": 0,
+      "positionLength": 2
+    },
+    {
+      "token": "Open",
+      "start_offset": 0,
+      "end_offset": 4,
+      "type": "<ALPHANUM>",
+      "position": 0
+    },
+    {
+      "token": "Search",
+      "start_offset": 4,
+      "end_offset": 10,
+      "type": "<ALPHANUM>",
+      "position": 1
+    },
+    {
+      "token": "helped",
+      "start_offset": 11,
+      "end_offset": 17,
+      "type": "<ALPHANUM>",
+      "position": 2
+    },
+    {
+      "token": "many",
+      "start_offset": 18,
+      "end_offset": 22,
+      "type": "<ALPHANUM>",
+      "position": 3
+    },
+    {
+      "token": "employers",
+      "start_offset": 23,
+      "end_offset": 32,
+      "type": "<ALPHANUM>",
+      "position": 4
+    }
+  ]
+}
+```