docs: arm: convert docs to ReST and rename to *.rst

Converts ARM the text files to ReST, preparing them to be an
architecture book.

The conversion is actually:
  - add blank lines and identation in order to identify paragraphs;
  - fix tables markups;
  - add some lists markups;
  - mark literal blocks;
  - adjust 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>
Reviewed-by Corentin Labbe <clabbe.montjoie@gmail.com> # For sun4i-ss

arch/arm/kernel/entry-armv.S

@@ -826,7 +826,7 @@ ENDPROC(__switch_to)
  * existing ones.  This mechanism should be used only for things that are
  * really small and justified, and not be abused freely.
  *
- * See Documentation/arm/kernel_user_helpers.txt for formal definitions.
+ * See Documentation/arm/kernel_user_helpers.rst for formal definitions.
  */
  THUMB(	.arm	)