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>

tools/testing/selftests/x86/protection_keys.c

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 /*
- * Tests x86 Memory Protection Keys (see Documentation/x86/protection-keys.txt)
+ * Tests x86 Memory Protection Keys (see Documentation/core-api/protection-keys.rst)
  *
  * There are examples in here of:
  *  * how to set protection keys on memory