Documentation/gpu: limit main toctree depth to 2

Documentation/gpu: limit main toctree depth to 2

The main GPU documentation toctree has no limit to the toctree depth,
which means the main GPU index page recursively includes all the
headings in all of GPU documentation in the single table of
contents. This makes getting any kind of overview of the documentation
really difficult.

Limit the main toctree depth to 2 i.e. show at most two levels of
headings.

Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://patch.msgid.link/cddd2d11c104132801510e3ab4e4b9ef3ea9cb6d.1778238671.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>