docs/sp_SP: Add translation of process/contribution-maturity-model

Translate Documentation/process/contribution-maturity-model.rst into
Spanish

Signed-off-by: Avadhut Naik <avadhut.naik@amd.com>
Reviewed-by: Carlos Bilbao <carlos.bilbao@amd.com>
[ jc: fix added warning for nonexistent ref ]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230717181602.3468421-1-avadhut.naik@amd.com