[Feature] Add guidance to use .NET Standard instead of PCL

[BillWagner]

PCLs are less important now that .NET Standard 2.0 is coming.

We need to update our guidance to recommend .NET Standard - based libraries instead of creating PCLs.

[lextm]

I think this initiative should be carried out as soon as possible.

Documentation on PCL, such as this and this should be marked as obsolete.