Overhaul Pack/Push documentation to prioritize modern package authoring

[zkat]

Reading through our documentation on package authoring is pretty confusing these days, because it's largely written with packages.config and .nuspec scenarios in mind. I've also noticed that a lot of documentation related to doing certain tasks in SDK-style projects is just plain missing.

We should comb through the docs and overhaul them to represent the way we do things now, while deprioritizing (but still documenting) the .nuspec/non-SDK scenarios.

[nkolev92]

There's already a duplicate doc issue here: https://github.com/NuGet/docs.microsoft.com-nuget/issues/2165 created by you :D

[zkat]

oh lol. I closed the other one, since this one will be more visible in general.