Integrate information about nullable reference types in relevant topics

Our current docs on C# 8 nullable reference types (NRT) exists in the [Required and Optional Properties](https://docs.microsoft.com/en-us/ef/core/modeling/required-optional), and in a dedicated new [Working with Nullable Reference Types](https://docs.microsoft.com/en-us/ef/core/miscellaneous/nullable-reference-types) page. As NRT is a cross-cutting concern, some/most of the comments in the NRT-specific page actually belong in various other articles (how to specify DbSets in your DbContext, how to query...).

After this is done, we can possibly link from the dedicated page to those pages, and/or maintain some level of duplication.

See original discussion in https://github.com/aspnet/EntityFramework.Docs/pull/1709#discussion_r324489339.