docs: move x86 documentation into Documentation/arch/

Move the x86 documentation under Documentation/arch/ as a way of cleaning
up the top-level directory and making the structure of our docs more
closely match the structure of the source directories it describes.

All in-kernel references to the old paths have been updated.

Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: linux-arch@vger.kernel.org
Cc: x86@kernel.org
Cc: Borislav Petkov <bp@alien8.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/lkml/20230315211523.108836-1-corbet@lwn.net/
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

arch/x86/include/asm/bootparam_utils.h

@@ -38,7 +38,7 @@ static void sanitize_boot_params(struct boot_params *boot_params)
 	 * IMPORTANT NOTE TO BOOTLOADER AUTHORS: do not simply clear
 	 * this field.  The purpose of this field is to guarantee
 	 * compliance with the x86 boot spec located in
-	 * Documentation/x86/boot.rst .  That spec says that the
+	 * Documentation/arch/x86/boot.rst .  That spec says that the
 	 * *whole* structure should be cleared, after which only the
 	 * portion defined by struct setup_header (boot_params->hdr)
 	 * should be copied in.