Create github action that files a ticket if documents get stale

[aarongable]

We are required to update each of these documents at least once every 365 days. We should have a github action which runs periodically (monthly?), checks the last time the documents were modified, and files a ticket if either of them is more 9 months old.

[pgporada]

Where should the complaints be filed?

[aarongable]

Filing a ticket in this repo would be ideal, but IIRC doing so requires the Action having a Github Personal Access Token, which we don't want to maintain. Sending an email is okay, but doesn't have any way to do duplicate-detection and prevent sending the same reminder multiple times. Maybe we go as far as to add a "stale document" prober type to boulder-observer, and just export a "time since last update" metric which can be alerted on via normal means? Oof.