Doc: Allow to easily link to qt_add_grpc() and qt_add_protobuf()

Add qt_add_grpc() and qt_add_protobuf() as alternative \keyword
entries to their respective CMake command pages, replacing the
\keyword qt6_add_protobuf entry.

Update the existing \l and \sa references elsewhere in the
documentation and examples to use the new keywords directly.

Pick-to: 6.12
Change-Id: I3f1a8df5e6ab4dfba02cf1043a66c01ed6a8c788
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>