EF Core: Provide guidance on what collection types to use

This is probably a subtopic in [relationships](https://github.com/aspnet/EntityFramework.Docs/blob/master/entity-framework/core/modeling/relationships.md).

EF Core is pretty flexible re the type of collection you use, so this is about describing what is supported and how to choose between all the types supported in different scenarios, like when you need an ordered list, data binding, lazy loading proxies, etc.

See the discussion in https://github.com/aspnet/EntityFramework.Docs/pull/1003.