docs: tox -e docs tests building the docs

tox.ini

@@ -2,13 +2,14 @@
 # fakeroot -u tox --recreate
 
 [tox]
-envlist = py{39,310,311}-{none,fuse2,fuse3}
+envlist = py{39,310,311}-{none,fuse2,fuse3},docs
 minversion = 3.2
 requires =
 	pkgconfig
 	cython
 	wheel
 	setuptools_scm
+
 [testenv]
 package = editable-legacy  # without this it does not find setup_docs when running under fakeroot
 setenv =
@@ -42,3 +43,11 @@ deps =
     mypy
     pkgconfig
 commands = mypy
+
+[testenv:docs]
+changedir = docs
+deps =
+    sphinx
+    guzzle_sphinx_theme
+commands =
+    sphinx-build -n -v -W --keep-going -b html -d {envtmpdir}/doctrees . {envtmpdir}/html