docs: build docs in CI (and be nitpicky about it)

This ensures that any warning generated from compiling the docs is treated as
an error. Also, building the docs is now one of the steps performed in CI.

.github/workflows/test.yml

@@ -29,3 +29,7 @@ jobs:
         run: make test-format
       - name: Package tests
         run: make test-pythonpackage
+      - name: Install docs dependencies
+        run: pip install -r requirements/docs.txt
+      - name: Build docs
+        run: make docs

docs/Makefile

@@ -2,7 +2,7 @@
 .PHONY: help
 
 html:
-	sphinx-build -b html -a -E "." "_build/html"
+	sphinx-build -b html -a -E -n -W "." "_build/html"
 
 browse:
 	sensible-browser _build/html/index.html