improve documentation on aggregation

It would be good to expand the Altair docs for [aggregation ](https://altair-viz.github.io/user_guide/transform/aggregate.html) with more info from [vl-aggregate-ops](https://vega.github.io/vega-lite/docs/aggregate.html#ops) and [vl-aggregate-argmax](https://vega.github.io/vega-lite/docs/aggregate.html#argmax).