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

The kbuild documentation clearly shows that the documents
there are written at different times: some use markdown,
some use their own peculiar logic to split sections.

Convert everything to ReST without affecting too much
the author's style and avoiding adding uneeded markups.

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>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/kernel-hacking/hacking.rst

@@ -718,7 +718,7 @@ make a neat patch, there's administrative work to be done:
 -  Usually you want a configuration option for your kernel hack. Edit
    ``Kconfig`` in the appropriate directory. The Config language is
    simple to use by cut and paste, and there's complete documentation in
-   ``Documentation/kbuild/kconfig-language.txt``.
+   ``Documentation/kbuild/kconfig-language.rst``.
 
    In your description of the option, make sure you address both the
    expert user and the user who knows nothing about your feature.
@@ -728,7 +728,7 @@ make a neat patch, there's administrative work to be done:
 
 -  Edit the ``Makefile``: the CONFIG variables are exported here so you
    can usually just add a "obj-$(CONFIG_xxx) += xxx.o" line. The syntax
-   is documented in ``Documentation/kbuild/makefiles.txt``.
+   is documented in ``Documentation/kbuild/makefiles.rst``.
 
 -  Put yourself in ``CREDITS`` if you've done something noteworthy,
    usually beyond a single file (your name should be at the top of the