docs: arm64: convert docs to ReST and rename to .rst

The documentation is in a format that is very close to ReST format.

The conversion is actually:
  - add blank lines in order to identify paragraphs;
  - fixing tables markups;
  - adding some lists markups;
  - marking literal blocks;
  - adjust some title markups.

At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.

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

arch/arm64/kernel/kexec_image.c

@@ -53,7 +53,7 @@ static void *image_load(struct kimage *image,
 
 	/*
 	 * We require a kernel with an unambiguous Image header. Per
-	 * Documentation/arm64/booting.txt, this is the case when image_size
+	 * Documentation/arm64/booting.rst, this is the case when image_size
 	 * is non-zero (practically speaking, since v3.17).
 	 */
 	h = (struct arm64_image_header *)kernel;