Set nitpicky mode in conf.py

And remove superfluous `verify_sphinx_doc.yml` workflow: this is checked by the ReadTheDocs integration

Author: maarten-ic

.github/workflows/verify_sphinx_doc.yml

@@ -34,6 +34,9 @@
 author = "ITER Organization"
 src_host = "https://github.com/iterorganization/"
 
+# Warn about missing references
+nitpicky = True
+
 # Parse urls here for convenience, to be re-used
 # ITER docs
 iter_projects = "https://github.com/iterorganization/"