Build(deps): Bump json from 2.8.2 to 2.9.0 in /docs

Bumps [json](https://github.com/ruby/json) from 2.8.2 to 2.9.0.
- [Release notes](https://github.com/ruby/json/releases)
- [Changelog](https://github.com/ruby/json/blob/master/CHANGES.md)
- [Commits](ruby/json@v2.8.2...v2.9.0)

---
updated-dependencies:
- dependency-name: json
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

docs/Gemfile.lock

@@ -211,7 +211,7 @@ GEM
       gemoji (>= 3, < 5)
       html-pipeline (~> 2.2)
       jekyll (>= 3.0, < 5.0)
-    json (2.8.2)
+    json (2.9.0)
     kramdown (2.4.0)
       rexml
     kramdown-parser-gfm (1.1.0)