Add .readthedocs.yml file to use python3.8

Specify build.image to latest in .readthedocs.yml file

Include all git submodules

.readthedocs.yml

@@ -0,0 +1,21 @@
+# .readthedocs.yaml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+version: 2
+
+# Build documentation in the docs/ directory with Sphinx
+sphinx:
+   configuration: doc/conf.py
+
+build:
+  image: latest
+
+# Include all submodules
+submodules:
+  include: all
+
+python:
+   version: "3.8"
+   install:
+   - requirements: doc/requirements.txt