doc: add a readthedocs config file

See https://blog.readthedocs.com/migrate-configuration-v2/

, config file
copy/pasted from there.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

.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/ directory with Sphinx
+sphinx:
+  configuration: doc/source/conf.py
+
+# We recommend specifying your dependencies to enable reproducible builds:
+# https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
+# python:
+#   install:
+#   - requirements: docs/requirements.txt
+