Add documentation on disabled runner registration token

What does this MR do and why?

Documents the new settings that allow admins and group owners to allow deprecated runner registration tokens.

The settings documented have been added by:

• Add UI to allow disabling use of registration t... (!147559 - merged)
• Adds "Allow runner registration token" setting ... (!148557 - merged)

MR acceptance checklist

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Related to #411917 (closed)