doc: improve picking example

- Fix background color
- Use columns for text
- Flip cone color
- Update image

Change-Id: I35baf863c6935b877681822377aeef0117f64fe6
Reviewed-by: Tomi Korpipää <tomi.korpipaa@qt.io>
Reviewed-by: Tony Leinonen <tony.leinonen@qt.io>
Reviewed-by: Andy Nichols <andy.nichols@qt.io>