Apart from the Windows-related notes, all instructions are common to our
releases and therefore do not need to be explicitly listed here. In
addition, most of the information was severely outdated, especially with
respect to how our packaging works.

We have dedicated repositories for packages, so this stuff is already
fragmented into visible and invisible. The docs can be fragmented the same
way. But then everything about this repo should be documented in this repo,
too. This just re-adds docs about the publicly visible stuff. Ex. the last
section, but it is easy to miss, so it's here.