[admin] Backend sections to be ported to SolidusAdmin

[elia]

We want to tackle the easy pages first, so we can slowly improve and augment our UI kit components and then reuse it to migrate more complex pages (e.g. promotions)

Follows a list of easy to migrate sections of Spree::Backend:

### List of low-hanging fruits
- [ ] Settings > Zones
- [ ] Settings > Shipping
- [ ] Settings > Returns & Refunds
- [ ] Settings > Taxes
- [ ] Settings > Payments
- [ ] Settings > Stores