build(deps): bump minitest from 5.14.4 to 5.15.0 in /docs

Bumps [minitest](https://github.com/seattlerb/minitest) from 5.14.4 to 5.15.0.
- [Release notes](https://github.com/seattlerb/minitest/releases)
- [Changelog](https://github.com/seattlerb/minitest/blob/master/History.rdoc)
- [Commits](https://github.com/seattlerb/minitest/compare/v5.14.4...v5.15.0)

---
updated-dependencies:
- dependency-name: minitest
  dependency-type: indirect
  update-type: version-update:semver-minor
...

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

docs/Gemfile.lock

@@ -240,7 +240,7 @@ GEM
       jekyll (>= 3.5, < 5.0)
       jekyll-feed (~> 0.9)
       jekyll-seo-tag (~> 2.1)
-    minitest (5.14.4)
+    minitest (5.15.0)
     mixlib-cli (2.1.8)
     mixlib-config (3.0.9)
       tomlrb