Document annotation mechanics

[hiddeco]

I have been wondering if we should start documenting some annotations as well, to allow people to drive certain mechanics (e.g. refreshes and reverifications) using kubectl.

This would both function as a way to explain how things work, in addition to providing documentation for the lowest level API available.

Originally posted by @hiddeco in https://github.com/akuity/kargo/issues/1869#issuecomment-2065407025