Explicitly configure ReadTheDocs build to use conf.py (pydata#9908)

I got an email indicating that this configuration will be required as
of January 20, 2025.

Author: shoyer

.readthedocs.yaml

@@ -1,5 +1,9 @@
 version: 2
 
+sphinx:
+  configuration: doc/conf.py
+  fail_on_warning: true
+
 build:
   os: ubuntu-lts-latest
   tools:
@@ -14,7 +18,4 @@ build:
 conda:
   environment: ci/requirements/doc.yml
 
-sphinx:
-  fail_on_warning: true
-
 formats: []