The documentation main page and the project home page have slightly
different functions. At the cost of slight duplication, have different
texts for each.

Add README.rst to conf.py excludes as it's only used by gitlab.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>