- Avoid caching RubyGems in GitHub Actions `tests.yml` to catch this in
  future.
- Run `brew doctor` in GitHub Actions `tests.yml` after installing
  RubyGems to catch this in future.
- Ignore relevant RubyGems (and fix outdated comments)
- Never auto-`require` RubyGems that aren't vendored.
- Update `bundler/setup.rb`.

- Add `--stackprof` to allow using stackprof
- Use the latest version of `ruby-prof`
- Automatically open the generated HTML output

Having HOMEBREW_PATCHELF_RB set in the ENV,
will conditionally install patchelf.rb gem,
use patchelf.rb in the above mentioned methods.
The installed vendored gems are listed in .gitignore
to maintain a clean state.

bundler/setup: add sorbet and sorbet-runtime

- Without this, vale won't correctly detect where the styles are.

This is the newest version that works with Ruby 2.3.

Also, update the `.gitignore` to hide it while we're here.

This allows maintaining more settings in one place.

This is a clearer branding for us to use.

Also, as part of testing this, make some fixes to Jekyll in the docs folder.

Fixes #5835

Exclude subdirectories and specific files. This makes adding/removing files on newly vendored versions much easier.

We aren't going to vendor its dependencies because we already require
a systemwide installation of `rubocop` to work in your editor. This
avoids requiring users to manually do another
`gem install rubocop-rspec` to have their editor integration behave as
expected.

Rather than having to manually keep track of what version each thing in
here is and copy files around by hand on update let's use Bundler's
standalone mode and careful use of `.gitignore` to help us do it.

This means a `bundle update --standalone` will allow us to update all
gems in vendor.

We could consider vendoring other gems this way in future but I'd
suggest only doing this for gems with no dependencies or at least gems
with no native extensions. The only gem this applies to that we
currently use is `ruby-prof` and I'm not convinced it's widely used
enough to warrant vendoring for everyone. Perhaps that's another
criteria: it should be functionality that's used by non-developer
commands and/or normal Homebrew usage.

Remove a bunch of unneeded, old directories and generally cleanup.