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

tox.ini

@@ -40,6 +40,12 @@ envdir = {toxworkdir}/venv
 commands =
   bash tools/config/generate_sample.sh -b . -p ironic -o etc/ironic
 
+[testenv:gendocs]
+sitepackages = False
+envdir = {toxworkdir}/venv
+commands =
+  python setup.py build_sphinx
+
 [testenv:venv]
 setenv = PYTHONHASHSEED=0
 commands = {posargs}