docs: m68k: convert docs to ReST and rename to *.rst

Convert the m68k kernel-options.txt file to ReST.

The conversion is trivial, as the document is already on a format
close enough to ReST. Just some small adjustments were needed in
order to make it both good for being parsed while keeping it on
a good txt shape.

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>