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/kernel/cpu/microcode/amd.c

@@ -61,7 +61,7 @@ static u8 amd_ucode_patch[MAX_NUMNODES][PATCH_MAX_SIZE];
 
 /*
  * Microcode patch container file is prepended to the initrd in cpio
- * format. See Documentation/x86/microcode.rst
+ * format. See Documentation/arch/x86/microcode.rst
  */
 static const char
 ucode_path[] __maybe_unused = "kernel/x86/microcode/AuthenticAMD.bin";