chore: Updating final Store docs for v3

markdown/dev/reference/api/store/extend/en.md

@@ -0,0 +1,48 @@
+---
+title: Store.extend()
+---
+
+The `Store.extend()` method can be used to extend the store with new
+functionality by passing an array of methods and the path to the location in
+the store where they should be attached. It is typically not used directly, but
+rather through a plugin.
+
+## Signature
+
+```js
+Store Store.extend(Array methods=[])
+```
+
+<Tip compact>This method is chainable as it returns the `Store` object</Tip>
+
+The single argument should be an Array of methods to add to the
+store. Each entry in the array should be an array itself holding a path in
+dot notation and a method, as such:
+
+```js
+function myCustomMethod() {
+ // Do something clever
+}
+
+const store = new Store([
+  ["path.to.the.method", myCustomMethod ]
+])
+```  
+
+With the configuration above, you can call `store.path.to.the.method()` and it
+will run `myCustomMethod()`.
+
+Stores the value of `value` in the store under key `key`.
+
+## Notes
+
+The Store will not allow you to extend any of the following keys:
+
+- `set`
+- `setIfUnset`
+- `push`
+- `unset`
+- `get`
+- `extend`
+
+