Add now mandatory readthedocs config file

Docs builds currently fail with:

```
Problem in your project's configuration. No default configuration file found at repository's root. See https://docs.readthedocs.io/en/stable/config-file/
```

See e.g. https://readthedocs.org/projects/requests/builds/22842479/

.readthedocs.yaml

@@ -0,0 +1,28 @@
+# Read the Docs configuration file for Sphinx projects
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+
+# Set the OS, Python version and other tools you might need
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.12"
+
+# Build documentation in the "docs/" directory with Sphinx
+sphinx:
+  configuration: docs/conf.py
+
+# Optionally build your docs in additional formats such as PDF and ePub
+formats:
+  - pdf
+  - epub
+
+# Optional but recommended, declare the Python requirements required
+# to build your documentation
+# See https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
+python:
+  install:
+    - path: .
+    - requirements: docs/requirements.txt