turn on fail-on-warning for Sphinx builds (ray-project#37651)

Signed-off-by: angelinalg <[email protected]>

.readthedocs.yaml

@@ -14,6 +14,7 @@ build:
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
  configuration: doc/source/conf.py
+ fail_on_warning: true
 
 # We recommend specifying your dependencies to enable reproducible builds:
 # https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html