doc: move documentation from the tex file to docbook

And remove the .tex file

Minor changes:
- where the .tex file had some interface descriptions, the docbook source
  now links to the actual protocol. The exception here is the shared object
  cache which is simply a <programlisting> until the protocol spec exists.
- "Implementation" section skipped, this seems in need of an update anyway
  and may be better documented elsewhere (wiki?)

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>