docs: add .readthedocs.yml to set Python version and requirements.txt

.readthedocs.yml

+# .readthedocs.yaml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+
+# Build documentation in the docs/ directory with Sphinx
+sphinx:
+  configuration: docs/conf.py
+
+# Optionally set the version of Python and requirements required to build your docs
+python:
+   version: 3.8
+   install:
+   - requirements: docs/requirements.txt