docs: adds migration guide for Assets<Mesh> retaining render_world only meshes

[kfc35]

This is targeting release-0.18.0, NOT main!

Objective

• Fixes 0.18: Potential panic when accessing Meshes due to retained render world only assets #22206

Solution

• Adds a migration guide for the changes in Retain asset without data for RENDER_WORLD-only assets #21732. As someone not too familiar with mesh stuff, I’d appreciate a review from someone more familiar with the changes and mesh things!

FYI @robtfm @beicause

[github-actions]

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
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[github-actions]

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
You can review it at https://pixel-eagle.com/project/B04F67C0-C054-4A6F-92EC-F599FEC2FD1D?filter=PR-22407

If it's expected, please add the M-Deliberate-Rendering-Change label.

If this change seems unrelated to your PR, you can consider updating your PR to target the latest main branch, either by rebasing or merging main into it.

[github-actions]

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
You can review it at https://pixel-eagle.com/project/B04F67C0-C054-4A6F-92EC-F599FEC2FD1D?filter=PR-22407

If it's expected, please add the M-Deliberate-Rendering-Change label.

If this change seems unrelated to your PR, you can consider updating your PR to target the latest main branch, either by rebasing or merging main into it.