build(deps-dev): bump github-pages from 219 to 220 in /docs

Bumps [github-pages](https://github.com/github/pages-gem) from 219 to 220.
- [Release notes](https://github.com/github/pages-gem/releases)
- [Commits](https://github.com/github/pages-gem/compare/v219...v220)

---
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

@@ -48,8 +48,8 @@ GEM
     ffi (1.15.4)
     forwardable-extended (2.6.0)
     gemoji (3.0.1)
-    github-pages (219)
-      github-pages-health-check (= 1.17.7)
+    github-pages (220)
+      github-pages-health-check (= 1.17.9)
       jekyll (= 3.9.0)
       jekyll-avatar (= 0.7.0)
       jekyll-coffeescript (= 1.1.1)
@@ -92,7 +92,7 @@ GEM
       nokogiri (>= 1.10.4, < 2.0)
       rouge (= 3.26.0)
       terminal-table (~> 1.4)
-    github-pages-health-check (1.17.7)
+    github-pages-health-check (1.17.9)
       addressable (~> 2.3)
       dnsruby (~> 1.60)
       octokit (~> 4.0)