Bump nokogiri from 1.13.1 to 1.13.4 in /docs

Bumps [nokogiri](https://github.com/sparklemotion/nokogiri) from 1.13.1 to 1.13.4.
- [Release notes](https://github.com/sparklemotion/nokogiri/releases)
- [Changelog](https://github.com/sparklemotion/nokogiri/blob/v1.13.4/CHANGELOG.md)
- [Commits](https://github.com/sparklemotion/nokogiri/compare/v1.13.1...v1.13.4)

---
updated-dependencies:
- dependency-name: nokogiri
  dependency-type: indirect
...

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

docs/Gemfile.lock

@@ -77,7 +77,7 @@ GEM
       rb-inotify (~> 0.9, >= 0.9.10)
     mercenary (0.4.0)
     minitest (5.15.0)
-    nokogiri (1.13.1-x86_64-linux)
+    nokogiri (1.13.4-x86_64-linux)
       racc (~> 1.4)
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)