Documentation/mm: Initial page table documentation

This is based on an earlier blog post at people.kernel.org,
it describes the concepts about page tables that were hardest
for me to grasp when dealing with them for the first time,
such as the prevalent three-letter acronyms pfn, pgd, p4d,
pud, pmd and pte.

I don't know if this is what people want, but it's what I would
have wanted. The wording, introduction, choice of initial subjects
and choice of style is mine.

I discussed at one point with Mike Rapoport to bring this into
the kernel documentation, so here is a small proposal.

The current form is augmented in response to feedback from
Mike Rapoport, Matthew Wilcox, Jonathan Cameron, Kuan-Ying Lee,
Randy Dunlap and Bagas Sanjaya.

Cc: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Mike Rapoport <rppt@kernel.org>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://people.kernel.org/linusw/arm32-page-tables


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230614072548.996940-1-linus.walleij@linaro.org