This is a good first (top) page to have in the docs, let's move it out
from the pipewire heading.
This doesn't change any of the links and the renaming isn't required
(subpage controls whether it's a top-level item and the meson.build
order decides its spot in he list). Still, better to have the filenames
represent the layout.
This is an attempt of breaking up the documentation, currently spread across
several pages. We're left with a few high-level topics with various things
grouped underneath those.
Further refinement is necessary, but this can now be done in incremental steps
over massive reworks.
