docs/vm: add vmalloced-kernel-stacks document

docs/vm: add vmalloced-kernel-stacks document

Add a new document to explain Virtually Mapped Kernel Stack Support.
This is a compilation of information from the code and original patch
series that introduced the Virtually Mapped Kernel Stacks feature.

This document summarizes the feature and provides details on allocation,
free, and stack overflow handling.  Provides reference to available
tests.

Link: https://lkml.kernel.org/r/20211215002004.47981-1-skhan@linuxfoundation.org
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>