Add mermaid diagram 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 a Mermaid graph to visualize the differences between the legacy registration method and the new one, in hopes of making the changes clearer for users landing on the blueprint document.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Review app: https://main-ee-111606.docs.gitlab-review.app/ee/architecture/blueprints/runner_tokens/index.html#comparison-of-current-and-new-runner-registration-flow

How to set up and validate locally

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

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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