raise nitpicky warnings as errors

.github/workflows/check_external_links.yml → .github/workflows/check_sphinx_links.yml

@@ -1,12 +1,12 @@
-name: Check Sphinx external links
+name: Check Sphinx links
 on:
   pull_request:
   schedule:
     - cron: '0 5 * * *'  # once per day at midnight ET
   workflow_dispatch:
 
 jobs:
-  check-external-links:
+  check-sphinx-links:
     runs-on: ubuntu-latest
     steps:
       - name: Cancel non-latest runs
@@ -31,5 +31,5 @@ jobs:
           python -m pip install -r requirements-doc.txt
           python -m pip install .
 
-      - name: Check Sphinx external links
-        run: sphinx-build -b linkcheck ./docs/source ./test_build
+      - name: Check Sphinx internal and external links
+        run: sphinx-build -W -b linkcheck ./docs/source ./test_build

docs/Makefile

@@ -149,7 +149,7 @@ changes:
 	@echo "The overview file is in $(BUILDDIR)/changes."
 
 linkcheck:
-	$(SPHINXBUILD) -b linkcheck $(ALLSPHINXOPTS) $(BUILDDIR)/linkcheck
+	$(SPHINXBUILD) -W -b linkcheck $(ALLSPHINXOPTS) $(BUILDDIR)/linkcheck
 	@echo
 	@echo "Link check complete; look for any errors in the above output " \
 	      "or in $(BUILDDIR)/linkcheck/output.txt."

docs/make.bat

@@ -183,7 +183,7 @@ if "%1" == "changes" (
 )
 
 if "%1" == "linkcheck" (
-	%SPHINXBUILD% -b linkcheck %ALLSPHINXOPTS% %BUILDDIR%/linkcheck
+	%SPHINXBUILD% -W -b linkcheck %ALLSPHINXOPTS% %BUILDDIR%/linkcheck
 	if errorlevel 1 exit /b 1
 	echo.
 	echo.Link check complete; look for any errors in the above output ^