Add documentation for playbook names

[dmsimard]

It's possible to give playbook names by supplying the ara_playbook_name Ansible variable.

The intent of the feature is to allow users to give their playbook user friendly names that would display in-lieu of the playbook path by default.
For example, you might have 3 different playbooks named "pre" "run" and "post".

The name is a searchable field in the API and users could therefore search for all the playbooks named "run", for example.

[dmsimard]

This will land with https://review.opendev.org/#/c/661040/

[dmsimard]

Merged: https://ara.readthedocs.io/en/latest/playbook-names-and-labels.html