Add missing actions to data store documentation (#11986)

* Add missing actions to the documentation

- Add the missing actions to the following:

- data-store/cart
- data-store/checkout
- data-store/collections
- data-store/validation

* Move actions above selectors in Cart doc

* Add more info to cart actions

* Fix syntax of Cart actions examples

* Fix syntax and examples

- Use store name constant instead of strings
- Fix description and examples of collections document

* Add more info about the keys of objects and arrays

* Update billing address info in cart data

* Update parameter names and descriptions in
checkout.md and collections.md

* Update docs/third-party-developers/extensibility/data-store/checkout.md

Co-authored-by: Niels Lange <info@nielslange.de>

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Co-authored-by: Niels Lange <info@nielslange.de>