Add sphinx.configuration to .readthedocs.yml (elastic#2740) (elastic#2744)

(cherry picked from commit 1f66af5)

Co-authored-by: Quentin Pradet <[email protected]>
(cherry picked from commit 2aaacf5)

# Conflicts:
#	.readthedocs.yml

Author: github-actions[bot]

.readthedocs.yml

@@ -13,3 +13,6 @@ python:
   install:
     - requirements: dev-requirements.txt
     - path: .
+
+sphinx:
+  configuration: docs/sphinx/conf.py