Documentation: change default highlight language to shell ...

There are many places where sphinx complains that blocks cannot
be hilghlighted in 'c' and in fact I am not aware of any C code
snippets in the documentation, so change the default highlighting
style to shell which fits our needs much better. With this we
get rid of many warnings while building the documentation.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Documentation: Add new sphinxs docs ...

This is a rewrite of the Documentation in reStructuredText format using
Sphinx as build system, see http://sphinx-doc.org/.

The documentation is built into static html pages with 'make docs'.
The pages can be found under Documentation/html after building.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>