Documentation/writing_documentation.md: Explain how to use docker

Using docker to build to documentation eases the process of building
the documentation. Given that some versions of sphinx are
incompatible, the option to use docker is presented first.

Change-Id: I6c18f81a829364ada1859c04ba2dc4f886934bcc
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36105
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/getting_started/writing_documentation.md

@@ -14,7 +14,29 @@ coreboot uses [Sphinx] documentation tool. We prefer the markdown format
 over reStructuredText so only embedded ReST is supported. Checkout the
 [Markdown Guide] for more information.
 
-### Install Sphinx
+### option 1: Use the docker image
+
+The easiest way to build the documentation is using a docker image.
+To build the image run the following in the base directory:
+
+	make -C util/docker/ doc.coreboot.org
+
+Before building the documentation make sure the output directory is given
+the correct permissions before running docker.
+
+	mkdir -p Documentation/_build
+
+To build the documentation:
+
+	make -C util/docker docker-build-docs
+
+To have the documentation build and served over a web server live run:
+
+	make -C util/docker docker-livehtml-docs
+
+On the host machine, open a browser to the address <http://0.0.0.0:8000>.
+
+### option 2: Install Sphinx
 
 Please follow this official [guide] to install sphinx.
 You will also need python-recommonmark for sphinx to be able to handle