Added .readthedocs.yaml

Based it on the Sphinx example config file at
https://docs.readthedocs.io/en/stable/config-file/v2.html.

This should fix the recent build fail (see
https://readthedocs.org/projects/django-simple-history/builds/21202330/),
which was due to using a module that was added in Python 3
(`importlib.metadata`) with Python 2.7 - which is the version that's for
some reason being used to build the docs.

Author: ddabble

.readthedocs.yaml

@@ -0,0 +1,23 @@
+---
+# 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: "3.11"
+
+# Build documentation in the "docs/" directory with Sphinx
+sphinx:
+  configuration: docs/conf.py
+
+# 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:
+    - requirements: requirements/docs.txt