Improve review re-request email structure, layout, and design

What does this MR do and why?

For #429861 (closed)

Updates the email that informs a reviewer that a new review request has been started with a number of tweaks per the issue above.

• New heading for the type of email ("Review Requested")
• Re-word of copy + expanded info in that section
• Summary section remains the same
• Add CTA button to go to the overview page
• Remove emphasis from names in additional info line

Screenshots or screen recordings

	Before	After
with LLM
without LLM

How to set up and validate locally

1. View /rails/mailers/notify/request_review_merge_request_email.html to see the email.

MR acceptance checklist

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