Add tests docs to CI and fix docs warning

tox.ini

@@ -22,6 +22,14 @@ commands=
     python {toxinidir}/manage.py makemigrations --check --dry-run
     python -Wd manage.py test -v2 {posargs}
 
+[testenv:docs]
+deps=
+    sphinx
+    -r{toxinidir}/rtd_requirements.txt
+commands=make html clean SPHINXOPTS="-W --keep-going"
+changedir={toxinidir}/docs
+allowlist_externals=make
+
 [testenv:flake8]
 deps=flake8
 commands=flake8 django_mailbox