Add FAQ section to Next Runner Token Architecture blueprint

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR adds the start of an FAQ section to the bottom of the Next Runner Token Architecture blueprint, to help users navigate the upcoming changes.

Screenshots or screen recordings

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Review app: https://main-ee-111674.docs.gitlab-review.app/ee/architecture/blueprints/runner_tokens/index.html#faq

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.