Document stale bot and other community automations in maintainers guide

We have a stale bot, but we haven't documented its operation. Let's do it :)

Related: https://github.com/nsidc/earthaccess/discussions/895

https://jacobtomlinson.dev/posts/2024/most-stale-bots-are-anti-user-and-anti-contributor-but-they-dont-have-to-be/