ci(docs): RTD Sphinx warnings false successes

Use the same options on readthedocs.com that we use locally to fail on all warnings,
mostly to better catch stale cross references.

.readthedocs.yaml

@@ -7,6 +7,7 @@ build:
 
 sphinx:
   configuration: docs/source/conf.py
+  fail_on_warning: true
 
 python:
   install:
@@ -14,4 +15,4 @@ python:
     - method: pip
       path: .
       extra_requirements:
-        - docs
+        - docs