Adding new page to explain sys-audio setup.

[GammaSQ]

This is basically me taking notes during my latest sys-audio setup after a Qubes 4.2 installation. It's not very refined, but I took care to document every single step. Please have a good look at it, especially the policy-rules! I still get warnings of sys-audio trying to access other things, but those don't seem to be fatal?

I'd love to have an official statement on what permissions sys-audio needs (and why).

I made no mention of the salt-command, as it always breaks my system and causes weird side-effects.

Furthermore, I deliberately avoided discussing security-implications, as I am too inexperienced to say anything substantive.