docs: add fail_on_warning to Read the Docs config

* This is needed since Read the Docs doesn't use the repo Makefile.

.readthedocs.yml

@@ -14,6 +14,7 @@ build:
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   configuration: docs/conf.py
+  fail_on_warning: true
 
 # Optionally build your docs in additional formats such as PDF and ePub
 formats: all