docs/mm: Physical Memory: Populate the "Zones" section

Briefly describe what zones are and the fields of struct zone.

Link: https://lkml.kernel.org/r/20250315211317.27612-1-jiwen7.qi@gmail.com


Signed-off-by: Jiwen Qi <jiwen7.qi@gmail.com>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>