Add "Register runner" documentation page for new process

What does this MR do and why?

This change adds the page where new runner creation workflow is documented.

See: #396282 (closed)

As initially this functionality is only available to administrators, this page indicates that but the page will hold more documentation about the runner creation moving forward.

Note: This page will be be added to our docs navigation yet, I'll add a separate MR for this at: gitlab-docs!3655 (closed)

Screenshots or screen recordings

Please see this feature in action at: !113557 (merged)

How to set up and validate locally

NA

MR acceptance checklist

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