Make it easier for users to create autoscaled runners

ZD: https://gitlab.zendesk.com/agent/tickets/32110

Customer attempted to use the registration token in the admin/runners endpoint for autoscaled runners. This is an easy mistake to make: a GitLab CI registration token can be mistaken for a runner token. To make autoscaled runners work, he has to register 1 runner by hand first.

How do we make this easier? Some ideas:

• If the runner token is not there, should we automatically try the registration token, and if that matches exchange a new runner token?
• Allow the admin to create a runner token from the page itself
• Make it clear in the documentation that registration token != runner token