Freshness monitoring

It's currently hard to know whether content is current.

My proposal:
- Each page should have a public-visible "last updated by [github handle]"
- We should have some script and automation so the tech writer has easy access to a report of which content ought to be reviewed soon, based on things like:
- not fresh
- commonly accessed (no point prioritizing dead content)
- tech writer should be able to update the verification with some expiration when they want to re-review
- tech writer needs a way to identify the Subject Matter Expert (SME) for any given page, so they know who to ask

https://www.notion.com/help/guides/verify-knowledge-your-teammates-can-trust-with-page-verification is an example of a system that does some of this.

No files, tests, or entry points are named. Start by clarifying the requirements for page verification, review reporting, expiration, and SME identification, using the linked Notion example as context. Done should be defined by an agreed design and implementation scope for these capabilities.

Written by the indexing model from the issue text.