Explain properly that elementary OS can be used as a bluetooth speaker

## Problem

I had forgotten this feature even existed until Cassidy mentioned it on twitter - I don't even know if it's mentioned anywhere

## Proposal

Add some text below the main header and text saying something like "This device can be used as a bluetooth audio out" with a little [i] next to it that when hovered over explains that devices that can stream bluetooth audio to other devices can connect to elementary OS and stream music to it.