Hello,

I'm working on a plugin that creates temporary files that should be cleaned up at the end.
i'm having an issue caused by on_post_build firing multiple times (as it should) during mkdocs serve.
I'd like to propose an on_cleanup event type that runs once and only once at the end of either mkdocs build or mkdocs serve.

i'm happy to take a shot at implementing this first but would like some feedback on the idea before i try.