Add readthedocs.yml config file. (#711)

Make previously (grandfathered) ReadTheDocs configuration explicit. This is in
preparation for upgrading to Python 3 which is the default on ReadTheDocs now.

[ci skip]

Author: m-renaud

.readthedocs.yml

@@ -0,0 +1,11 @@
+version: 2
+
+sphinx:
+  configuration: containers/docs/conf.py
+
+submodules:
+  include:
+    - containers/docs/_extensions/haddock-autolink
+
+python:
+  version: 2.7