Set the default highlight language to "none", i.e. do not try to guess
the language and do automatic syntax highlighting on literal blocks.

Eyeballing around the generated documentation, we don't seem to actually
have a lot of literal blocks that would benefit from syntax
highlighting. The C code blocks we do have are typically very short, and
most of the literal blocks are things that shouldn't be highlighted (or,
do not have a pygments lexer). This seems to be true for literal blocks
both in the rst source files and in source code comments.

Not highlighting code is never wrong, but guessing the language wrong
almost invariably leads to silly or confusing highlighting.

At the time of writing, admin-guide/oops-tracing.rst and
admin-guide/ramoops.rst contain good examples of 1) a small C code
snippet not highlighted, 2) a hex dump highligted as who knows what, 3)
device tree block highlighted as C or maybe Python, 4) a terminal
interaction highlighted as code in some language, and finally, 5) some C
code snippets correctly identified as C. I think we're better off
disabling language guessing, and going by explicitly identified
languages for longer code blocks.

It is still possible to enable highlighting on an rst source file basis
using the highlight directive:

.. higlight:: language

and on a literal block basis using the code-block directive:

.. code-block:: language

See http://www.sphinx-doc.org/en/latest/markup/code.html

 for details.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Cc: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>