Documentation/arm64: Add ptdump documentation

Documentation/arm64: Add ptdump documentation

ptdump is a debugfs interface used to dump the kernel page tables. It
provides a comprehensive overview about the kernel's virtual memory
layout, page table entries and associated page attributes. A document
detailing how to enable ptdump in the kernel and analyse its output has
been added.

Changes in V2:

- Corrected command to cat /sys/kernel/debug/kernel_page_tables

Changes in V1:

https://lore.kernel.org/all/20230613064845.1882177-1-chaitanyas.prakash@arm.com/

Cc: Will Deacon <will@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
CC: linux-arm-kernel@lists.infradead.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Chaitanya S Prakash <chaitanyas.prakash@arm.com>
Link: https://lore.kernel.org/r/20230619083802.76092-1-chaitanyas.prakash@arm.com
[catalin.marinas@arm.com: various minor fixups; sorted index.rst in alphabetical order]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>