docs: move protection-keys.rst to the core-api book

This document is used by multiple architectures:

	$ echo $(git grep -l  pkey_mprotect arch|cut -d'/' -f 2|sort|uniq)
	alpha arm arm64 ia64 m68k microblaze mips parisc powerpc s390 sh sparc x86 xtensa

So, let's move it to the core book and adjust the links to it
accordingly.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/core-api/index.rst

@@ -34,6 +34,7 @@ Core utilities
    timekeeping
    boot-time-mm
    memory-hotplug
+   protection-keys
 
 
 Interfaces for kernel debugging