build: bump html-proofer from 3.11.0 to 3.11.1 in /docs

Bumps [html-proofer](https://github.com/gjtorikian/html-proofer) from 3.11.0 to 3.11.1.
- [Release notes](https://github.com/gjtorikian/html-proofer/releases)
- [Commits](https://github.com/gjtorikian/html-proofer/compare/v3.11.0...v3.11.1

)

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

docs/Gemfile.lock

@@ -84,7 +84,7 @@ GEM
     html-pipeline (2.11.0)
       activesupport (>= 2)
       nokogiri (>= 1.4)
-    html-proofer (3.11.0)
+    html-proofer (3.11.1)
       activesupport (>= 4.2, < 6.0)
       addressable (~> 2.3)
       mercenary (~> 0.3.2)