Tutorial: generics, nullable constraints and nullable attributes

In this tutorial, explain how to annotate a generic class, and a set of generic methods using the `notnull` constraints, and related attributes: 

- `AllowNull`
- `DisallowNull`
- `MaybeNull`, `MaybeNullWhen(bool)`
- `NotNull`, `NotNullWhen(bool)`
- `NotNullIfNotNull(string)`