EF Core 2.1: GroupBy translation documentation

[divega]

I am not sure whether we should create a new page or section for GroupBy, or just snippets for the what's new in EF Core 2.1 page. We currently don't have our own documentation for specific query operators.

We can discuss cases that can be translated, which would be related to https://github.com/aspnet/EntityFramework.Docs/blob/master/entity-framework/core/querying/client-eval.md.

Let's discuss it.

[divega]

Talking to @smitpatel again about this, we agreed that it is probably good idea to have a topic in the docs explaining the differences between GroupBy LINQ operator and GROUP BY clause in SQL and the cases that we support in EF Core.

The description of dotnet/efcore#2341 can be a starting point.