Add docs to tox.ini (mozillaGH-36)

tox.ini

@@ -5,6 +5,7 @@ envlist =
     {3.8,3.9,pypy3.9}-4.0.x
     {3.6,3.7,3.8,3.9,pypy3}-3.2.x
     {3.6,3.7,3.8,3.9,pypy3}-2.2.x
+    docs
 
 [testenv]
 setenv =
@@ -27,4 +28,12 @@ deps=
     3.2.x: Django>=3.2,<3.3
     4.0.x: Django>=4.0,<4.1
     main: https://github.com/django/django/archive/main.tar.gz
-    4.1.x: https://github.com/django/django/archive/1d071ec1aa8fa414bb96b41f7be8a1bd01079815.tar.gz 
+    4.1.x: https://github.com/django/django/archive/1d071ec1aa8fa414bb96b41f7be8a1bd01079815.tar.gz
+
+[testenv:docs]
+allowlist_externals = make
+basepython = python3
+skip_install = true
+commands = make -C docs html
+deps=
+    sphinx