Update lazy loading documentation for 3.0 changes

Let's get a draft of the docs for this feature in the 2.1.0-preview1 time frame. We either add the contents into the existing section in [related data](https://github.com/aspnet/EntityFramework.Docs/blob/master/entity-framework/core/querying/related-data.md#lazy-loading) or create a new page if it is too long.