Bump webrick from 1.8.1 to 1.9.1 in /docs

Bumps [webrick](https://github.com/ruby/webrick) from 1.8.1 to 1.9.1.
- [Release notes](https://github.com/ruby/webrick/releases)
- [Commits](ruby/webrick@v1.8.1...v1.9.1)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

docs/Gemfile.lock

@@ -263,7 +263,7 @@ GEM
       concurrent-ruby (~> 1.0)
     unicode-display_width (1.8.0)
     uri (0.13.0)
-    webrick (1.8.1)
+    webrick (1.9.1)
 
 PLATFORMS
   x86_64-linux