build(deps-dev): bump github-pages from 220 to 221 in /docs

Bumps [github-pages](https://github.com/github/pages-gem) from 220 to 221.
- [Release notes](https://github.com/github/pages-gem/releases)
- [Commits](https://github.com/github/pages-gem/compare/v220...v221)

---
updated-dependencies:
- dependency-name: github-pages
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

docs/Gemfile.lock

@@ -22,7 +22,7 @@ GEM
     em-websocket (0.5.2)
       eventmachine (>= 0.12.9)
       http_parser.rb (~> 0.6.0)
-    ethon (0.14.0)
+    ethon (0.15.0)
       ffi (>= 1.15.0)
     eventmachine (1.2.7)
     execjs (2.8.1)
@@ -48,7 +48,7 @@ GEM
     ffi (1.15.4)
     forwardable-extended (2.6.0)
     gemoji (3.0.1)
-    github-pages (220)
+    github-pages (221)
       github-pages-health-check (= 1.17.9)
       jekyll (= 3.9.0)
       jekyll-avatar (= 0.7.0)
@@ -89,7 +89,7 @@ GEM
       liquid (= 4.0.3)
       mercenary (~> 0.3)
       minima (= 2.5.1)
-      nokogiri (>= 1.10.4, < 2.0)
+      nokogiri (>= 1.12.5, < 2.0)
       rouge (= 3.26.0)
       terminal-table (~> 1.4)
     github-pages-health-check (1.17.9)