Markdown cache upgrade counter clarifications

What does this MR do and why?

• Adds a link to the Markdown Cache Version Upgrades dashboard to the rollout docs.
• Fix up Don't increment Markdown version upgrade counte... (!242638 - merged)'s mistaken assumption: those writes we saw before enabling the rollout weren't from new writes, they're actually legitimate old cache upgrades!
  • #save_markdown starts by immediately returning if the record is persisted?, which is why they couldn't possibly be from new writes.
  • Document it in code a little more clearly, explain the necessity of the cached_markdown_version_in_database.nil? check (it's not for new records, it's for weird ones!), correct the spec added in !242638 (merged), add a new one for clarity.
• Count upgrades and backfills, and count them separately!
  • "Weird" records per above count as a backfill; anything which isn't an upgrade, and could be seen even outside of a rollout window.
• Heavily refactor the specs for readability and verifiability.

References

• Prevent markdown version changes from impacting... (#330313 - closed)
• Add means to bump Markdown cache gradually (!236200 - merged)
• Bumping CACHE_COMMONMARK_VERSION is risky (#597379 - closed)

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