Docs/process/index: Remove unsorted docs section

'Other material' section on 'process/index' is no more necessary since
we have 'staging/' directory.  Also all documents on the section has
moved to better places.  Remove the section.

Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240624185312.94537-6-sj@kernel.org