Expose `scale_bar` (and other overlays) in API docs

> After yesterday's community meeting discussion this is a reminder that the API does not contain info on scale_bar options. @TimMonko says that the sphinx contribution needs to be explicitly updated 

 _Originally posted by @MargotCh in [#38](https://github.com/napari/docs/issues/38#issuecomment-3817688775)_