docs: promote the title of process/index.rst

...otherwise Sphinx won't cooperate when trying to list it explicitly in
the top-level index.rst file

Reviewed-by: David Vernet <void@manifault.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20220927160559.97154-2-corbet@lwn.net
Signed-off-by: Jonathan Corbet <corbet@lwn.net>