Add readthedocs configuration file

Our documentation builds are failing with the following error and I
think there is no way to specify the package dependency in the web UI.

    No package 'dbus-1' found

.readthedocs.yaml

@@ -0,0 +1,17 @@
+# .readthedocs.yaml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+version: 2
+
+sphinx:
+   configuration: doc/source/conf.py
+
+build:
+  image: latest
+  apt_packages:
+    - libdbus-1-dev
+
+python:
+   install:
+   - requirements: requirements.txt