Update docs about changing routes

What does this MR do and why?

This updates our docs to better reflect our stance on changing routes. That is, keep redirects indefinitely.

History

In %16.0, we removed some legacy redirects. Even though many of them had been in place for four major releases before they were removed, we encountered lots of problems with our own scripts and documents 404ing, and heard from the community similar problems.

So, we reverted the removal of the redirects.

Screenshots or screen recordings

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How to set up and validate locally

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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.