Doc: Allow to easily link to Wayland CMake commands

Add qt_generate_wayland_protocol_server_sources() as an alternative
\keyword to the CMake command page, replacing
\keyword qt6_generate_wayland_protocol_server_sources.

Update the existing \l references in the compositor overview and
examples to use the new keyword directly.

Pick-to: 6.12
Change-Id: Ice13dd8db4886bed1a97a0beda34ec0c80efd10a
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>