Tox: Adds docs target

tox.ini

@@ -49,3 +49,9 @@ setenv = C_DEBUG_TEST = 1
 commands = {toxinidir}/extra/release/removepyc.sh {toxinidir}
            pip install -U -r{toxinidir}/requirements/dev.txt
            nosetests -xsv --with-coverage --cover-inclusive --cover-erase []
+
+[testenv:docs]
+deps = -r{toxinidir}/requirements/docs.txt
+commands =
+    pip install -U -r{toxinidir}/requirements/dev.txt
+    sphinx-build -W -b linkcheck -d {envtmpdir}/doctrees docs docs/_build/linkcheck