Format rate limiting phases to markdown list

What does this MR do and why?

Formats the Phases and Iterations section of the Rate Limiting Blueprint to a Markdown list for improved readability.

Current version in the docs: https://docs.gitlab.com/ee/architecture/blueprints/rate_limiting/#phases-and-iterations

Screenshots or screen recordings

Current Formatting

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.