WIP: Add docs for API and webhooks

guides/api.md

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+---
+layout: default
+title: Engine API
+has_children: false
+nav_order: 6
+parent: Guides
+permalink: /guides/api/
+---
+
+{% include page_header.html %}
+
+## API
+Engine offers a REST API for interacting with areas of the store. While the API
+does not touch every area, it offers access to the most-used resources. Our API
+is built atop the Solidus API, so the documentation for that API
+(https://solidus.docs.stoplight.io/) is a great place to start. Beyond the base
+Solidus API, we have also added the following resource endpoints:
+
+* `/api/subscriptions`: create and update subscriptions

guides/webhooks.md

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+---
+layout: default
+title: Webhooks
+has_children: false
+nav_order: 7
+parent: Guides
+permalink: /guides/webhooks/
+---
+
+{% include page_header.html %}
+
+## Webhooks
+
+In addition to our REST API, we also offer a webhooks interface for clients who
+need more interactive access to events occurring on the store. Our webhooks are
+based on the REST Hooks convention (https://resthooks.org/). We support the
+following hook events within Engine:
+
+* `carton_shipped`: occurs when a carton is marked as shipped.
+* `order_cancelled`: occurs when an order is marked as cancelled
+* `order_completed`: occurs when a customer completes the checkout process for an order
+* `order_inventory_cancelled`: occurs when an item on an order is cancelled from that order
+* `product_updated`: occurs when a product's information changes
+* `product_viewed`: occurs when a product page is viewed
+* `reimbursement_failed`: occurs when a return reimbursement fails to process
+* `reimbursement_succeeded`: occurs when a return reimbursement processes successfully