content: add documentation for programmatically focusing in ion-app

[brandyscarney]

URL

https://ionicframework.com/docs/api/app

Issue Description

The ion-app component has an internal setFocus method that can be used to programmatically focus elements. We should document this usage in the ion-app documentation and remove the @internal flag:

/**
 * @internal
 * Used to set focus on an element that uses `ion-focusable`.
 * Do not use this if focusing the element as a result of a keyboard
 * event as the focus utility should handle this for us. This method
 * should be used when we want to programmatically focus an element as
 * a result of another user action. (Ex: We focus the first element
 * inside of a popover when the user presents it, but the popover is not always
 * presented as a result of keyboard action.)
 */
@Method()
async setFocus(elements: HTMLElement[]) {
  if (this.focusVisible) {
    this.focusVisible.setFocus(elements);
  }
}

We may want to link to this documentation from other components to show how to programmatically focus them.

Related issue: ionic-team/ionic-framework#29830