doc: Comprehensive user & contributor guides

Created by: valschneider

The LISA transition guide is a cleaned up version of the changelog I had written when the next branch was first created. It's probably missing some bits (we did change a lot of things), do point them out. Also, if you've struggled with some specific points, do bring it up so we can make sure others don't go through the same pain. You can have a preview of it here: https://my-lisa.readthedocs.io/en/lisa-next/users_guide.html#transitioning-from-lisa-legacy

Contributor's guide needs some filling - just enough to get people going with subtrees and that should be it.