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>

Documentation/admin-guide/kernel-parameters.rst

@@ -118,7 +118,7 @@ parameter is applicable::
 	LOOP	Loopback device support is enabled.
 	M68k	M68k architecture is enabled.
 			These options have more detailed description inside of
-			Documentation/m68k/kernel-options.txt.
+			Documentation/m68k/kernel-options.rst.
 	MDA	MDA console support is enabled.
 	MIPS	MIPS architecture is enabled.
 	MOUSE	Appropriate mouse support is enabled.

Documentation/m68k/index.rst

+:orphan:
+
+=================
+m68k Architecture
+=================
+
+.. toctree::
+   :maxdepth: 2
+
+   kernel-options
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`