Add a Back to Ansible Documentation button to nav

[gotmax23]

The DAWGs is discussing providing a way for ecosystem docsites to link back to the main site. See ansible/ansible-documentation#877. It would be nice to integrate this into the mkdocs-ansible theme and have a always present link in the navbar. Perhaps something like this?

[ssbarnea]

While I do see the use for a back to main, the current implementation looks bit ugly, too intrusive. Maybe not having the inverted background would make it more subtle?

[gotmax23]

Something more toned down works for me.