Updates #7: Updating Advanced/Interface/External audio

Hello @Garf, This commit updates the documentation on how to play external audio over a video, updating from https://wiki.videolan.org/VLC_HowTo/Play_an_external_audio_track_for_a_video/ as mentioned in #7

This is also an old MR which got closed by me by mistake. The latest development in this was the issue I was facing in audio/video track switching, I will raise a ticket for it soon.