Documentation: Add support for building with Sphinx

This commit adds the necessary infrastructure to convert the Markdown
files in the Documentation directory to HTML using Sphinx[1] and
recommonmark[2]. I selected "sphinx_rtd_theme" as the theme, because it
offers a useful navigation sidebar, and because it's already used for
the Linux kernel[3].

Makefile.sphinx was auto-generated by sphinx-quickstart. conf.py was
auto-generated and manually adjusted.

[1]: http://www.sphinx-doc.org/en/stable/
[2]: https://recommonmark.readthedocs.io/en/latest/
[3]: https://www.kernel.org/doc/html/latest/index.html

Change-Id: Ie4de96978e334c598cf5890775807d3e15c29c4d
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/25787
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

.gitignore

@@ -132,5 +132,6 @@ documentation/cpukconfig.tex
 documentation/mainboardkconfig.tex
 documentation/skconfig.tex
 documentation/socketfkconfig.tex
+Documentation/_build
 
 doxygen/*