docs: ZENKO-2639 add .readthedocs.yml file spec

As recommended by readthedocs, we're adding the proper
yaml spec file to specify the configurations to build the
documentation.

It also provides build time env variable that will help us
identify in the build if we are building with readthedocs or not.

.readthedocs.yml

@@ -0,0 +1,19 @@
+# .readthedocs.yml
+# 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/docsource/conf.py
+
+# Optionally build your docs in additional formats such as PDF
+formats: []
+
+# Optionally set the version of Python and requirements required to build your docs
+python:
+  version: 3.7
+  install:
+    - requirements: docs/docsource/requirements.txt