The docs have several "main sections" on the left
One of those sections is "Aggregation"
Under that menu item you can find "Aggregation Reference" which has some subcategories that allows you to see some of the available operators.
Under "Reference" however, there is a "Operators" menu item and then "Aggregation Framework Operators" that give you this list of possibilities.
This highly confusing no matter on which section you start at.
I would like the entire http://docs.mongodb.org/master/reference/operator/aggregation/ page to be moved to http://docs.mongodb.org/master/core/aggregation-introduction/