docs/mm: Physical Memory: add structure, introduction and nodes description

Add structure, introduction and Nodes section to Physical Memory
chapter.

As the new documentation references core-api/dma-api and mm/page_reclaim,
add page labels to those documents.
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
Link: https://lore.kernel.org/r/20230125192841.25342-2-rppt@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/core-api/dma-api.rst

+.. _dma_api:
+
 ============================================
 Dynamic DMA mapping using the generic device
 ============================================

Documentation/mm/page_reclaim.rst

 .. SPDX-License-Identifier: GPL-2.0
 
+.. _page_reclaim:
+
 ============
 Page Reclaim
 ============