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

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

The conversion here is trivial:
 - Adjust the document title's markup
 - Do some whitespace alignment;
 - mark literal blocks;
 - Use ReST way to markup indented lists.

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>