build: bump html-proofer from 3.12.2 to 3.13.0 in /docs

Bumps [html-proofer](https://github.com/gjtorikian/html-proofer) from 3.12.2 to 3.13.0.
- [Release notes](https://github.com/gjtorikian/html-proofer/releases)
- [Commits](https://github.com/gjtorikian/html-proofer/compare/v3.12.2...v3.13.0

)

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

docs/Gemfile.lock

@@ -84,7 +84,7 @@ GEM
     html-pipeline (2.12.0)
       activesupport (>= 2)
       nokogiri (>= 1.4)
-    html-proofer (3.12.2)
+    html-proofer (3.13.0)
       addressable (~> 2.3)
       mercenary (~> 0.3)
       nokogiri (~> 1.10)