Document about Gemfile.lock

### 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

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

### Issue

When you install locally, using `bundle install` you get one version. And GitHub Pages will always use its own version.

GitHub Pages will never use your `Gemfile.lock` to resolve dependencies when building your GitHub Pages website.

This fact is important for developers using the `pages-gem` gem as well as users of GitHub Pages in general.

RELATED: https://github.com/github/gitignore/pull/3690

### Follow on work

Add this documentation as justification at: https://github.com/github/gitignore/pull/3690