docs: put atomic*.txt and memory-barriers.txt into the core-api book

docs: put atomic*.txt and memory-barriers.txt into the core-api book

These files describe part of the core API, but have never been converted to
RST due to ... let's say local oppposition.  So, create a set of
special-purpose wrappers to ..include these files into a separate page so
that they can be a part of the htmldocs build.  Then link them into the
core-api manual and remove them from the "staging" dumping ground.

Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Reviewed-by: David Vernet <void@manifault.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20220927160559.97154-7-corbet@lwn.net
Signed-off-by: Jonathan Corbet <corbet@lwn.net>