docs: add .readthedocs.yml conf file to pin Python dependencies

.readthedocs.yaml

@@ -0,0 +1,21 @@
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+
+build:
+  os: ubuntu-20.04
+  tools:
+    python: "3.9"
+
+sphinx:
+   configuration: docs/conf.py
+
+formats:
+  - pdf
+  - epub
+
+python:
+   install:
+   - requirements: docs/requirements.txt

docs/requirements.txt

@@ -0,0 +1,2 @@
+sphinx==4.0.2
+sphinx_rtd_theme==0.5.2