Document what font should be used

### Problem

I am creating an app. And I don't really know what fonts I should use in the app.
This isn't covered anywhere in the documentation as far as I can see.

### Proposal

It would be nice if the [text section](https://docs.elementary.io/hig/reference/text) of the documentation talked a bit about:

* what font should be used
* what is the recommended size
* are they CSS class that should be used

### Prior Art (Optional)

The [iconography](https://docs.elementary.io/hig/reference/iconography) does a good job of documenting how things should look like.