build(deps): bump rainbow from 3.0.0 to 3.1.1 in /docs

Bumps [rainbow](https://github.com/sickill/rainbow) from 3.0.0 to 3.1.1.
- [Release notes](https://github.com/sickill/rainbow/releases)
- [Changelog](https://github.com/sickill/rainbow/blob/master/Changelog.md)
- [Commits](https://github.com/sickill/rainbow/compare/v3.0.0...v3.1.1)

---
updated-dependencies:
- dependency-name: rainbow
  dependency-type: indirect
  update-type: version-update:semver-minor
...

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

docs/Gemfile.lock

@@ -265,7 +265,7 @@ GEM
       forwardable-extended (~> 2.6)
     public_suffix (4.0.6)
     racc (1.6.0)
-    rainbow (3.0.0)
+    rainbow (3.1.1)
     rake (13.0.6)
     rb-fsevent (0.11.0)
     rb-inotify (0.10.1)