Document aggregate functions

[roji]

We should have a general conceptual section for aggregate functions, possibly under complex query operators (though technically aside from Min/Max and friends these aren't exactly LINQ operators).

• Generally explain how to you use them in the projection after GroupBy
• Show different supported operators (distinct, ordering, filter)
• Show workaround for executing these top-level, with GroupBy on a constant (see https://github.com/dotnet/efcore/issues/28264#issue-1275817597)
• Link from the translation reference to this section.
• Mention that aggregate UDF mapping isn't yet possible (https://github.com/dotnet/efcore/issues/27934)

[roji]

Oh oops, of course there's already a GroupBy section with some details.

We could add the above to it, though we need to talk about top-level aggregate too.