attempt to use tox to build the documentation

tox.ini

@@ -14,3 +14,10 @@ commands =
     py{27,36}: coverage run --append --source=src/icalendar --omit=*/tests/* --module pytest [] src/icalendar/tests/hypothesis/
     coverage report
     coverage html
+
+[testenv:docs]
+deps =
+    -r {toxinidir}/requirements_docs.txt
+commands =
+    cd docs
+    make html