Clearer paths for custom platforms, versions and partner reviews

New guide: Which API version do I use? Core and Reviews are moving from V1 to V3. Loyalty stays on V2, with a few V3 endpoints.
Custom platform with Reviews and Loyalty: the recipe is now Custom platform: sync orders to Reviews and Loyalty. Send every order to both Core Create order (review requests) and Loyalty Create Order (points). Both endpoint pages now say so.
Which token endpoint: Generate access token is for integrating one store. Apps that many merchants install use the app partners token.
App partners: Pull a store's reviews into your app now includes unpublished reviews, so you can hide a review the merchant unpublishes.
Redeeming points: Redeem points for a reward shows the not-enough-points error, which uses amount instead of errors.