Add read the docs config file version 2

The config file is by now necessary to build documentation:
https://blog.readthedocs.com/migrate-configuration-v2/

.readthedocs.yaml

@@ -0,0 +1,18 @@
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+version: 2
+
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.11"
+
+# Build documentation in the docs/ directory with Sphinx
+sphinx:
+  configuration: docs/conf.py
+
+# https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
+# python:
+#  install:
+#  - requirements: docs/requirements.txt