🚑 Make Sphinx nitpicky about broken references

Fixes #2424

docs/conf.py

@@ -146,3 +146,7 @@ link_files = {
         ],
     ),
 }
+
+
+# Be strict about any broken references:
+nitpicky = True

tox.ini

@@ -49,7 +49,16 @@ extras =
         testing
 changedir = docs
 commands =
-        python -m sphinx . {toxinidir}/build/html
+        {envpython} -m sphinx \
+          -j auto \
+          -b html \
+          --color \
+          -a \
+          -n \
+          -W \
+          -d "{temp_dir}/.doctrees" \
+          . \
+          "{toxinidir}/build/html"
 
 [testenv:finalize]
 skip_install = True