Always use a distro~release subdirectory for output, cache, builddir

[DaanDeMeyer]

Let's be consistent and always use a distro~release subdirectory for these directories, instead of only using these when the correct directory exists in the directory mkosi is invoked from.

[DaanDeMeyer]

Added a blurb to news, let's keep it out of the manpage though I'd say, seems a bit too detailed to mention there, more of an implementation detail.

[behrmann]

Agreed for the cache and builddir, but on the output directory it is userfacing. I mean, it's not like this will confuse users, but it is something they'll see.

[DaanDeMeyer]

Only docs updates, so let's not wait for CI