CI: check yard & mdl output

[EricFromCanada]

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An attempt at having CI test for issues with YARD's rubydoc generation. Doesn't currently work because I'm unsure of how to get bundler working. (Next step after that would be to add Markdown formatting checks using mdl now that it's shipped a release with this fix, although it now requires Ruby 2.7.)

[BrewTestBot]

Review period will end on 2022-11-18 at 19:55:46 UTC.

[Bo98]

We no longer expose our install of Bundler at all and there's no dev-cmd entry point beyond what brew vendor-gems offers at the moment.

Though a Bundler installation from an external Ruby via ruby/setup-ruby is probably sufficient. Will probably install gems to the vendor directory given the .bundle/config but that's ok since we're not committing anything here.

If any Bundler 2.3 is available (which I think ruby/setup-ruby might provide for Ruby 2.6, 2.7 and 3.1), Bundler will actually auto-download and install the version of Bundler specified in the lockfile.

[BrewTestBot]

Review period ended.

[EricFromCanada]

Seems to be working now. For now it won't hold up any future PRs until these steps are enabled:

• "Check Markdown syntax": pending some final fixes for the last few docs.brew.sh sections (coming soon)
• "Process rubydoc comments": needs someone to track down how to either fix the error and two warnings that YARD produces, or have them be skipped

[MikeMcQuaid]

Great work here! If/when you merge this: please open another draft PR with the broken YARD stuff and I'll attempt to figure it out and push to it.

[MikeMcQuaid]

I reckon it's maybe work splitting this into a separate job given it's working on a different Bundler? Thoughts?

[EricFromCanada]

I think you may be overestimating my knowledge of bundler, gems, and Ruby in general… 🙃

[MikeMcQuaid]

@EricFromCanada Sorry, I mean the two differing Gemfile/BUNDLE_GEMFILE bits here may make sense to be in different GitHub Actions jobs in the same file. Thoughts?

[EricFromCanada]

They're now in separate jobs. Only outstanding item is the Test Everything check which seems to be unrelated.

[MikeMcQuaid]

Thanks again @EricFromCanada!