C# reference consolidation: nullable value types

[pkulikov]

All three programming guide topics about nullable value types:
https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/nullable-types/

should be combined and moved as one article to the language reference part of the docs.

@BillWagner I can do that in October. If that's fine, please assign this to me.

[BillWagner]

Thanks @pkulikov I've assigned this to you.