build: bump html-pipeline from 2.12.0 to 2.12.1 in /docs

Bumps [html-pipeline](https://github.com/jch/html-pipeline) from 2.12.0 to 2.12.1.
- [Release notes](https://github.com/jch/html-pipeline/releases)
- [Changelog](https://github.com/jch/html-pipeline/blob/master/CHANGELOG.md)
- [Commits](https://github.com/jch/html-pipeline/compare/v2.12.0...v2.12.1

)

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

docs/Gemfile.lock

@@ -81,7 +81,7 @@ GEM
       octokit (~> 4.0)
       public_suffix (~> 3.0)
       typhoeus (~> 1.3)
-    html-pipeline (2.12.0)
+    html-pipeline (2.12.1)
       activesupport (>= 2)
       nokogiri (>= 1.4)
     html-proofer (3.13.0)