Bump nokogiri from 1.8.1 to 1.13.4 in /docs

Bumps [nokogiri](https://github.com/sparklemotion/nokogiri) from 1.8.1 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](sparklemotion/nokogiri@v1.8.1...v1.13.4)

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

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

Author: dependabot[bot]

docs/Gemfile.lock

@@ -190,19 +190,21 @@ GEM
       rb-fsevent (>= 0.9.3)
       rb-inotify (>= 0.9.7)
     mercenary (0.3.6)
-    mini_portile2 (2.3.0)
+    mini_portile2 (2.8.0)
     minima (2.1.1)
       jekyll (~> 3.3)
     minitest (5.11.1)
     multipart-post (2.0.0)
     net-dns (0.8.0)
-    nokogiri (1.8.1)
-      mini_portile2 (~> 2.3.0)
+    nokogiri (1.13.4)
+      mini_portile2 (~> 2.8.0)
+      racc (~> 1.4)
     octokit (4.8.0)
       sawyer (~> 0.8.0, >= 0.5.3)
     pathutil (0.16.1)
       forwardable-extended (~> 2.6)
     public_suffix (2.0.5)
+    racc (1.6.0)
     rb-fsevent (0.10.2)
     rb-inotify (0.9.10)
       ffi (>= 0.5.0, < 2)