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>

Documentation/index.md

@@ -0,0 +1,21 @@
+Welcome to coreboot's documentation!
+====================================
+
+This is the developer documentation for [coreboot](https://coreboot.org).
+It is built from Markdown files in the
+[Documentation](https://review.coreboot.org/cgit/coreboot.git/tree/Documentation)
+directory in the source code.
+
+Contents:
+
+* [Lesson 2: Submitting a patch to coreboot.org](Lesson2.md)
+* [Gerrit Etiquette and Guidelines](gerrit_guidelines.md)
+* [coreboot's build system](build_system.md)
+* [Kconfig in coreboot](core/Kconfig.md)
+* [Use of git submodules in coreboot](submodules.md)
+* [Timestamps](timestamp.md)
+* [Dealing with Untrusted Input in SMM](technotes/2017-02-dealing-with-untrusted-input-in-smm.md)
+* [ABI data consumption](abi-data-consumption.md)
+* [GPIO toggling in ACPI AML](acpi/gpio.md)
+* [Native Graphics Initialization with libgfxinit](gfx/libgfxinit.md)
+* [Sandy Bridge Raminit](Intel/NativeRaminit/Sandybridge.md)