Add a ReadTheDocs config for 4.9

Copied from 5.2

.github/.readthedocs.yaml

@@ -0,0 +1,25 @@
+# Read the Docs configuration file for Sphinx projects
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+
+build:
+  os: ubuntu-24.04
+  tools:
+    python: "3.12"
+  jobs:
+      post_create_environment:
+        - python -m pip install sphinx-rtd-theme
+
+# Build documentation in the "doc/" directory with Sphinx
+sphinx:
+  configuration: doc/conf.py
+  # Fail on all warnings to avoid broken references
+  # fail_on_warning: true
+
+# Optionally build your docs in additional formats such as PDF and ePub
+formats:
+   - pdf
+   - epub
+   - htmlzip