Updating Advanced/Player/Multiple Instances

Hello @Garf, I had been working on an MR but it contained someone else's commits too. I tried to fix it for a couple of hours but that made everything more complicated. I thought I would delete my fork, re-fork the original repo, redo today's work, and that would fix the issue. But, it was a bad decision in hindsight as it led to the closure of all my previous MRs as well.

I apologise for the inconvenience.

Amongst the wikis mentioned in #7, this commit updates "Playing Multiple Instances" of VLC (Advanced/Player). The methods have been grouped OS wise instead of Graphic/CI wise as it would be more helpful for the reader that way. Also, the macOS method was slightly confusing in the wiki (https://wiki.videolan.org/VLC_HowTo/Play_multiple_instances/), whose contents have been rearranged to make the flow easier.

I am also tagging fkuehne, as @Garf had in the previous MR. @fkuehne, will this fix #6?