Add gendocs tox job for generating the documentation

We already use tox to generate the sample configuration, run the tests,
etc.. This patch is adding a new 'gendocs' tox job for generating the
documentation. I always have a hard time remembering the syntax to
generate the docs, by adding it to tox I hope it may facilitate it a bit.

You can now (re)generate the docs with this tox job:

tox -egendocs

The quick start guide was also updated with the new command.

Change-Id: I6a755c31382310420d706e51a3e014941fd49872
Merge "Add gendocs tox job for generating the documentation"