Bump mcp from 0.20.0 to 0.21.0 in /docs

Bumps [mcp](https://github.com/modelcontextprotocol/ruby-sdk) from 0.20.0 to 0.21.0.
- [Release notes](https://github.com/modelcontextprotocol/ruby-sdk/releases)
- [Changelog](https://github.com/modelcontextprotocol/ruby-sdk/blob/main/CHANGELOG.md)
- [Commits](modelcontextprotocol/ruby-sdk@v0.20.0...v0.21.0)

---
updated-dependencies:
- dependency-name: mcp
  dependency-version: 0.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

docs/Gemfile.lock

@@ -169,7 +169,7 @@ GEM
       net-smtp
     marcel (1.1.0)
     matrix (0.4.2)
-    mcp (0.20.0)
+    mcp (0.21.0)
       json_schemer (>= 2.4)
     method_source (1.1.0)
     mini_mime (1.1.5)