Flaws found by content/auto-cleanup-bot should prevent the PR from being merged

Flaws found by the content/auto-cleanup-bot workflow should prevent the PR from being merged.

This would have prevented issues such that which was caused by https://github.com/mdn/content/pull/42124, which resulted in an empty sidebar being deployed to production (fixed via https://github.com/mdn/content/pull/42143/files).

@caugner has some context:

Not every flaw is the same:

• Some flaws can be fixed in content, some require changes to rari (to keep up with content changes; rari makes some assumptions).
• Some flaws are automatically fixed on build (afaik only redirects), and are fixed in content using an auto-fix workflow. (Afaik this decoupling is useful, partly because we don’t require PRs to be up-to-date, which would regress contributor experience.)
  [W]e can only [error on every flaw] with merge queues, because otherwise two concurrently merged PRs may cause flaws, even if each separately don’t have any.

We should start with flaws that we are sure will be problematic (eg. "unknown flaws"), and iterate over time.

Start by reading .github/workflows/auto-cleanup-bot.yml and reviewing content PRs 42124 and 42143 for the failure scenario. Then examine the issue's distinction between unknown flaws, auto-fixed flaws, and merge queues; done means the agreed class of problematic flaws prevents merging without regressing the stated contributor workflow.