Bump mcp from 0.15.0 to 0.18.0 in /docs (#404)

Bumps [mcp](https://github.com/modelcontextprotocol/ruby-sdk) from 0.15.0 to 0.18.0.
- [Release notes](https://github.com/modelcontextprotocol/ruby-sdk/releases)
- [Changelog](https://github.com/modelcontextprotocol/ruby-sdk/blob/main/CHANGELOG.md)
- [Commits](modelcontextprotocol/ruby-sdk@v0.15.0...v0.18.0)

---
updated-dependencies:
- dependency-name: mcp
  dependency-version: 0.18.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

docs/Gemfile.lock

@@ -103,8 +103,8 @@ GEM
       securerandom (>= 0.3)
       tzinfo (~> 2.0, >= 2.0.5)
       uri (>= 0.13.1)
-    addressable (2.8.7)
-      public_suffix (>= 2.0.2, < 7.0)
+    addressable (2.9.0)
+      public_suffix (>= 2.0.2, < 8.0)
     ast (2.4.3)
     base64 (0.3.0)
     bigdecimal (4.1.2)
@@ -166,7 +166,7 @@ GEM
       net-smtp
     marcel (1.1.0)
     matrix (0.4.2)
-    mcp (0.15.0)
+    mcp (0.18.0)
       json-schema (>= 4.1)
     method_source (1.1.0)
     mini_mime (1.1.5)
@@ -209,7 +209,7 @@ GEM
     psych (5.3.1)
       date
       stringio
-    public_suffix (6.0.1)
+    public_suffix (7.0.5)
     puma (8.0.1)
       nio4r (~> 2.0)
     racc (1.8.1)