Add config file for ReadTheDocs.org

https://docs.readthedocs.io/en/stable/config-file/v2.html

.readthedocs.yaml

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+# .readthedocs.yaml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+
+# Set the version of Python and other tools you might need
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.11"
+
+# Build documentation in the docs/source/ directory with Sphinx
+sphinx:
+  configuration: docs/source/conf.py
+  builder: dirhtml
+
+# If using Sphinx, optionally build your docs in additional formats such as PDF
+# formats:
+#    - pdf
+
+# Optionally declare the Python requirements required to build your docs
+# python:
+#   install:
+#   - requirements: docs/requirements.txt