doc: start documenting Xwayland

This is a rough intro to what Xwayland is and does, with just one
implementation detail so far (Window identification).

I paid no attention to formatting details, those can be polished in
follow-ups. I just want the prose out.

I also just quickly whacked up the diagram, would be happy to see
someone replace it with a nicer one. I just didn't have time to learn
dot for now.

v2:
- typo fix
- rephrase "talking to hardware" as "driving the displays"
- mention circular dependency in intro
- add section to explain rootless and rootful modes
- remove paragraph about Xwayland protocol usage
- move TBD part to the end under a new section header

v3:
- use "advantage" and "disadvantage" instead of "pro" and "con"
- slight rewording on rootful mode and rootless mode paragraphs
- removed the paragraph about the lack of shell and special Wayland
  protocol extensions
- removed the commented out list of ideas to write

v4:
- typo fixes pointed out by Yong

Cc: Olivier Fourdan <ofourdan@redhat.com>
Cc: Jonas Ådahl <jadahl@gmail.com>
Cc: Daniel Stone <daniel@fooishbar.org>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>