add doc requirements

Author: bckohan

doc/requirements.txt

@@ -6,4 +6,5 @@ sphinxcontrib-htmlhelp==2.0.1; python_version >= "3.5"
 sphinxcontrib-jsmath==1.0.1; python_version >= "3.5"
 sphinxcontrib-qthelp==1.0.3; python_version >= "3.5"
 sphinxcontrib-serializinghtml==1.1.5; python_version >= "3.5"
+sphinx-js==2.8; python_version >= "3.5"
 django-render-static==2.0.0