Fail docs build on warnings

Author: jerith

.github/workflows/test.yaml

@@ -36,7 +36,7 @@ jobs:
       - name: Test
         run: poetry run pytest
       - name: Build docs
-        run: poetry run sphinx-build -b html docs docs/_build/html
+        run: poetry run sphinx-build -b html docs docs/_build/html --fail-on-warning
 
   docker-image:
     needs: [test]

docs/Makefile

@@ -3,7 +3,7 @@
 
 # You can set these variables from the command line, and also
 # from the environment for the first two.
-SPHINXOPTS    ?=
+SPHINXOPTS    ?= --fail-on-warning
 SPHINXBUILD   ?= sphinx-build
 SOURCEDIR     = .
 BUILDDIR      = _build