Change page from "Mathematical Operators" to "Mathematical and Aggregate Operators"

* move toList, reduce, concat, toSortedList, toMap, and toMultiMap to that page
* deprecate aggregate in favor of reduce

[should I move takeLast, last, and takeLastBuffer to the Aggregate operators page? they don't operate on the aggregate, exactly, but they do wait until the source completes before emitting anything]