doc: Split libwayland-client and -server into different pages

This was suggested before to make it clearer that things like wl_display
are different objects in each of them. I made these into two appendixes
because the protocol spec was already an appendix.

Reviewed-by: Bryce Harrington <b.harrington@samsung.com>
[Bryce requested minor changes, not yet here.]

Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>