docs(bitcoin): document get_blockchain_info endpoint in bitcoin guide

[marc0olo]

Context

The dfinity/examples submodule was bumped to d4ea422 (commit 12e1f0d), which added a get_blockchain_info endpoint to the basic_bitcoin example (Rust).

Current state

docs/guides/chain-fusion/bitcoin.mdx already has a short mention of get_blockchain_info (around line 456) with a minimal Rust snippet using ic_cdk_bitcoin_canister::get_blockchain_info. It does not:

• show the full BlockchainInfo response fields (height, block_hash, timestamp, difficulty, UTXO count)
• include a Motoko equivalent
• link to the example

Task

Expand the existing get_blockchain_info section in docs/guides/chain-fusion/bitcoin.mdx to:

1. Show the complete BlockchainInfo response struct and what each field means
2. Add a Motoko tab alongside the existing Rust snippet
3. Link to the example: https://github.com/dfinity/examples/tree/master/rust/basic_bitcoin (implementation at src/service/get_blockchain_info.rs)

Source material

• .sources/examples/rust/basic_bitcoin/src/service/get_blockchain_info.rs — Rust implementation
• .sources/examples/rust/basic_bitcoin/README.md — documents the response fields
• The BlockchainInfo type is in ic-cdk-bitcoin-canister = "0.2"

[marc0olo]

Recreating with correct formatting.