chore(deps-dev): bump jekyll-readme-index from 0.3.0 to 0.4.0 in /docs (#9403)

Bumps [jekyll-readme-index](https://github.com/benbalter/jekyll-readme-index) from 0.3.0 to 0.4.0.
- [Release notes](https://github.com/benbalter/jekyll-readme-index/releases)
- [Changelog](https://github.com/benbalter/jekyll-readme-index/blob/master/CHANGELOG.md)
- [Commits](https://github.com/benbalter/jekyll-readme-index/compare/v0.3.0...v0.4.0)

---
updated-dependencies:
- dependency-name: jekyll-readme-index
  dependency-version: 0.4.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

docs/Gemfile.lock

@@ -100,8 +100,9 @@ GEM
     jekyll-i18n_tags (1.0.0)
     jekyll-optional-front-matter (0.3.3)
       jekyll (>= 3.0, < 5.0)
-    jekyll-readme-index (0.3.0)
+    jekyll-readme-index (0.4.0)
       jekyll (>= 3.0, < 5.0)
+      kramdown-parser-gfm (~> 1.1)
     jekyll-relative-links (0.7.0)
       jekyll (>= 3.3, < 5.0)
     jekyll-sass-converter (3.1.0)