Mention 24px icons for notifications

### Discussed in https://github.com/elementary/docs/discussions/104

<div type='discussions-op-text'>

<sup>Originally posted by **Suzie97** July  9, 2021</sup>
Hey, I've been working on an app for AppCenter and noticed that the docs don't mention the need for 24px icons. But other elementary designed apps do have a 24px icon. 

Is this a new change or a typo in the docs?</div>