docs(app): add setFocus to the documentation (#29916)

Issue number: resolves #29830

---------

## What is the current behavior?
The `setFocus` method on `ion-app` is marked internal.

## What is the new behavior?
Document the `setFocus` method as a way for developers to
programmatically focus elements.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

## Other information

The method isn’t new, it was just marked as internal, which prevented it
from being documented. I can mark this as a `feat` though if anyone
thinks it should be.

Related documentation PR:
https://github.com/ionic-team/ionic-docs/pull/3842

packages/angular/src/directives/proxies.ts

@@ -188,6 +188,7 @@ Shorthand for ionAlertDidDismiss.
 
 
 @ProxyCmp({
+  methods: ['setFocus']
 })
 @Component({
   selector: 'ion-app',