Docs - Switch from flatly to readthedocs theme

Signed-off-by: Jared Ledvina <jared@techsmix.net>

mkdocs.yml

@@ -5,7 +5,7 @@ repo_url: https://github.com/sensu/sensu-ansible
 site_description: An Ansible role to deploy a fully dynamic Sensu stack!
 site_author: Calum MacRae
 
-theme: flatly
+theme: readthedocs
 pages:
   - Home: index.md
   - Usage: