Add a variant migration page

[D8H]

This PR must be merge right before the extensions updates.

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
gdevelop-wiki	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jun 17, 2025 8:52am

[github-actions]

Images automagically compressed by Calibre's image-actions ✨

Compression reduced images by 49%, saving 238.25 KB.

Filename	Before	After	Improvement	Visual comparison
docs/gdevelop5/objects/custom-objects-prefab-template/migrate-to-variants/button-children.png	110.57 KB	54.19 KB	-51.0%	View diff
docs/gdevelop5/objects/custom-objects-prefab-template/migrate-to-variants/button-properties.png	58.65 KB	40.39 KB	-31.1%	View diff
docs/gdevelop5/objects/custom-objects-prefab-template/migrate-to-variants/resource-bar-continuous-children.png	101.44 KB	46.96 KB	-53.7%	View diff
docs/gdevelop5/objects/custom-objects-prefab-template/migrate-to-variants/resource-bar-unit-children.png	86.73 KB	38.13 KB	-56.0%	View diff
docs/gdevelop5/objects/custom-objects-prefab-template/migrate-to-variants/slider-children.png	99.75 KB	45.03 KB	-54.9%	View diff
docs/gdevelop5/objects/custom-objects-prefab-template/migrate-to-variants/variant-selector.png	8.04 KB	5.53 KB	-31.2%	View diff
docs/gdevelop5/objects/custom-objects-prefab-template/object-scene-properties.png	21.17 KB	17.88 KB	-15.5%	View diff

2105 images did not require optimisation.