Add tox target to build the documentation

Change-Id: Ia0fa3ec6fd63e8539ab26a8226bb9e7dee9ce1c9

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = manifests,build_plugin
+envlist = manifests,docs,build_plugin
 skipsdist = True
 
 [testenv]
@@ -17,6 +17,12 @@ commands =
     bundle install --path {toxinidir}/.bundled_gems
     bundle exec rake test
 
+[testenv:docs]
+changedir = {toxinidir}/docs
+whitelist_externals = make
+commands =
+    make clean html SPHINXOPTS=-W
+
 [testenv:build_plugin]
 changedir = {toxinidir}
 whitelist_externals =