Have a detailed documentation page on pmbootstrap, instead of using the wiki

I am going to do further work on pmb_recommends, and I've realized that I really don't know where to properly document the changes. Seems like part of the internal pmbootstrap docs are in https://wiki.postmarketos.org/wiki/UI_specific_package, some other in https://wiki.postmarketos.org/wiki/Build_internals, and user instructions spread around in multiple other places. There is also no good documentation on the pmbootstrap library functions, that might be very useful for others to use. So I'm wondering if we wouldn't be better off having the documentation generated from the code with sphinx or similar, which is extremely easy to deploy in gitlab.

I'm aware of #2024 (closed), but the approach and rationale there seem to have been quite different.