Fix: Broken docs generation on readthedocs.org

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 skipsdist = true
 usedevelop = true
-envlist = py{37,38,39,310,311}-{all,core}
+envlist = py{37,38,39,310,311}-{all,core}, docs
 
 [testenv]
 setenv =
@@ -21,3 +21,11 @@ commands =
     core: coverage run runtests.py allianceauth.authentication.tests.test_app_settings -v 2 --debug-mode
     all: coverage report -m
     all: coverage xml
+
+[testenv:docs]
+description = invoke sphinx-build to build the HTML docs
+basepython = python3.9
+deps = -r{toxinidir}/docs/requirements.txt
+install_command =
+commands =
+    sphinx-build -T -E -b html -d "{toxworkdir}/docs_doctree" -D language=en docs "{toxworkdir}/docs_out" {posargs}