When adding redirects, also search the marketing site

What does this MR do?

This came up from a slack discussion (internal) when a docs page linked under https://about.gitlab.com/pricing/premium/ was a 404.

It turns out that when we redirected the file in !80242 (merged), we didn't also update the marketing site with the new URL.

This MR adds some information to the redirects documentation.

Related issues

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
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If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

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If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

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