[Topic Entry] Subject: Blockchain (#5882)

* Add blockchain topic entry

* Update blockchain.md

* Update blockchain.md

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+---
+Title: 'Blockchain'
+Description: 'Blockchain is a decentralized ledger that securely records transactions, ensuring transparency, trust, and immutability without a central authority.'
+Codecademy Hub Page: 'https://www.codecademy.com/catalog/subject/blockchain'
+CatalogContent:
+  - 'rust-for-programmers'
+  - 'paths/computer-science'
+---
+
+**Blockchain** is a decentralized and distributed digital ledger that securely records transactions across multiple nodes in a network. It ensures data integrity through cryptographic techniques and transparency by allowing participants to access an immutable, shared history of transactions. By eliminating the need for a central authority, blockchain enables trust and collaboration in various applications, from cryptocurrencies to supply chain management.
+
+Blockchain’s origins trace back to 1991 when cryptographers Stuart Haber and W. Scott Stornetta introduced a system for timestamping digital documents. The technology gained prominence in 2008 with Satoshi Nakamoto’s creation of Bitcoin, the first decentralized cryptocurrency using blockchain as its backbone. Over time, its applications have expanded beyond cryptocurrencies to include smart contracts, supply chain management, and enterprise solutions.
+
+Key principles of Blockchain include:
+
+- **Decentralization**: No central authority; data is shared across nodes.
+- **Cryptographic Security**: Data integrity ensured by encryption.
+- **Consensus Mechanisms**: Agreement protocols like Proof of Work or Proof of Stake.
+
+## Types of Blockchains
+
+1. **Public Blockchains**:
+   Open and decentralized networks where anyone can participate, read, or write data. These blockchains prioritize transparency and security, making them ideal for cryptocurrencies (e.g., Bitcoin, Ethereum). However, they may face scalability challenges and require significant energy for consensus.
+
+2. **Private Blockchains**:
+   Permissioned networks with restricted access, used by organizations to enhance efficiency and control. Only authorized participants can interact with the network, making it suitable for use cases like supply chain management or internal data sharing (e.g., Hyperledger, Corda).
+
+3. **Consortium Blockchains**:
+   Blockchains managed collaboratively by a group of organizations. These hybrid systems strike a balance between decentralization and controlled access, often used in industries where shared authority is required, such as banking or healthcare (e.g., R3 Corda, Quorum).