docs: explicitly reference mkdocs.yml in readthedocs config (#14016)

Signed-off-by: Tim Collins <[email protected]>

.readthedocs.yml

@@ -2,6 +2,7 @@ version: 2
 formats: all
 mkdocs:
   fail_on_warning: false
+  configuration: mkdocs.yml
 python:
   install:
     - requirements: docs/requirements.txt