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 <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/core-api/index.rst

···
    timekeeping
    boot-time-mm
    memory-hotplug
+   protection-keys
 
 
 Interfaces for kernel debugging

Documentation/x86/index.rst

···
    tlb
    mtrr
    pat
-   protection-keys
    intel_mpx
    amd-memory-encryption
    pti

arch/powerpc/Kconfig

···
 	  page-based protections, but without requiring modification of the
 	  page tables when an application changes protection domains.
 
-	  For details, see Documentation/vm/protection-keys.rst
+	  For details, see Documentation/core-api/protection-keys.rst
 
 	  If unsure, say y.
 

arch/x86/Kconfig

···
 	  page-based protections, but without requiring modification of the
 	  page tables when an application changes protection domains.
 
-	  For details, see Documentation/x86/protection-keys.txt
+	  For details, see Documentation/core-api/protection-keys.rst
 
 	  If unsure, say y.
 

tools/testing/selftests/x86/protection_keys.c

···
 // 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