Add 'webrick' warning note to "Quickstart" Docs (#8727)

Merge pull request 8727

docs/_docs/index.md

@@ -41,6 +41,9 @@ bundle exec jekyll serve
 ```
 6. Browse to [http://localhost:4000](http://localhost:4000){:target="_blank"}
 
+{: .note .warning}
+If you are using Ruby version 3.0.0 or higher, step 5 [may fail](https://github.com/github/pages-gem/issues/752). You may fix it by adding `webrick` to your dependencies: `bundle add webrick`
+
 {: .note .info}
 Pass the `--livereload` option to `serve` to automatically refresh the page with each change you make to the source files: `bundle exec jekyll serve --livereload`