Notable changes for theme developers to document for v9

This issue lists all the things necessary to migrate your theme from v8 to v9. We will start documenting those when we add v9 documentation.

## Brands page refacto
- [ ] documented

https://github.com/PrestaShop/classic-theme/pull/112
https://github.com/PrestaShop/PrestaShop/pull/31309

## Something else
- [ ] documented

link
link