Update docs or dependencies to include installing `webrick`

With a fresh install on mac using chruby, ruby `ruby 3.1.3p185 (2022-11-24 revision 1a6b16756e) [arm64-darwin21]` (as suggested by jekyll docs) and github-pages (227) the command for `jekyll serve` fails complaining about missing `webrick`. Adding this to the Gemfile fixes the issue.

### Before submitting an issue, please be sure to

- [x] Read [the contributing instructions](https://github.com/github/pages-gem/blob/master/docs/CONTRIBUTING.md#contributing-to-the-github-pages-gem)
- [x] Update to the latest Gem version (run `bundle update github-pages`)

### This issue affects

- [ ] The site generated by GitHub Pages
- [x] Building sites locally

*The GitHub Pages Gem is intended to help users replicate the GitHub Pages build environment locally. If your issue affects both the hosted version and the version previewed locally, you may be better suited reporting [seeking support in other forums](https://github.com/github/pages-gem/blob/master/docs/SUPPORT.md).*

### What did you do (e.g., steps to reproduce)
Followed the docs
### What did you expect to happen?
`jekyll serve` to work
### What happened instead?
This error https://github.com/jekyll/jekyll/issues/8523

### Additional information

* Link to the live site (if applicable):
* Link to the source repo (if applicable): https://github.com/yuletide/yuletide.github.io