Add reference documentation for DSC.PackageManagement/Apt

[Gijsreyn]

PR Summary

As part of #665, I thought the Linux users also would need some love. The resource manifest is reformatted.

PR Context

[Gijsreyn]

@michaeltlombardi I have removed the version for now as #930.

[SteveL-MSFT]

I want to keep in mind that the apt (and probably brew) resources will be rewritten in Python once we have that adapter. The current bash script was only intended as a demo. I also think we want to borrow some of the resource modeling from WinGet while also exposing some of the more commonly used capabilities of apt and brew. So I expect both of these resources to change substantially in the future when we get around to it.

[Gijsreyn]

I want to keep in mind that the apt (and probably brew) resources will be rewritten in Python once we have that adapter. The current bash script was only intended as a demo. I also think we want to borrow some of the resource modeling from WinGet while also exposing some of the more commonly used capabilities of apt and brew. So I expect both of these resources to change substantially in the future when we get around to it.

Understood, I added the small default remark that the resource is a proof-of-concept.