Fix RTD build by including config file. (#2274)

This is now required as layed out here:
  https://blog.readthedocs.com/migrate-configuration-v2/

Work towards #2267.

.readthedocs.yaml

@@ -0,0 +1,15 @@
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+version: 2
+
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.11"
+
+sphinx:
+  configuration: docs/conf.py
+
+python:
+  install:
+  - requirements: docs/rtd/requirements.txt