docs: add MCP Server documentation to fundamentals

[bloxster]

Summary

Adds a new MCP Server page to the gitbook documentation under the Fundamentals section (after Modules), documenting Erigon's built-in Model Context Protocol server.

What is covered

• Introduction to MCP: what the protocol is, why it matters, that it is read-only
• Two server variants: embedded (inside erigon via --mcp.addr/--mcp.port) vs standalone (cmd/mcp binary)
• Configuration tables: flags for both the embedded and standalone modes
• Three connection modes (standalone): JSON-RPC proxy, direct datadir, auto-discovery
• Use cases with examples:
  • Interactive blockchain analysis (querying balances, logs, block data via natural language)
  • Node debugging and monitoring (log triage, sync status checks)
• Claude Desktop setup: tabbed examples for JSON-RPC mode, datadir mode, and embedded SSE mode
• Tools reference: grouped listing of all 40+ tools (eth_*, erigon_*, ots_*, logs_*, metrics_*)
• Resources and Prompts tables: all erigon:// resource URIs and named prompt templates

Files changed

• docs/gitbook/src/fundamentals/mcp.md — new page
• docs/gitbook/src/SUMMARY.md — added * [MCP Server](fundamentals/mcp.md) after the Modules entry

[JkLondon]

btw now as I know by default erigon starts with mcp on 8553

[JkLondon]

tbh it's better if you test it, because it could be wrong
for me

{
  "mcpServers": {
    "erigon": {
      "command": "npx",
      "args": ["-y", "supergateway", "--sse", "http://localhost:8553/sse"]
    }
  }
}

works fine, other gateway providers don't (for some reason claude doesn't have native sse support, only stdin)