docs/mm: Physical Memory: remove useless markup

Jon says:

  > +See also :ref:`Page Reclaim <page_reclaim>`.

  Can also just be "See also Documentation/mm/page_reclaim.rst".  The
  right things will happen in the HTML output, readers of the plain-text
  will know immediately where to go, and we don't have to add the label
  clutter.

Remove reference markup and unnecessary labes and use plain file names.

Fixes: 5d8c5e43 ("docs/mm: Physical Memory: add structure, introduction and nodes description")
Suggested-by: Jonathan Corbet <corbet@lwn.net>
Acked-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
Link: https://lore.kernel.org/r/20230201094156.991542-2-rppt@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>