improve documentation on aggregation

[mattijn]

It would be good to expand the Altair docs for aggregation with more info from vl-aggregate-ops and vl-aggregate-argmax.

[tempdata73]

I'm gonna be tackling this in the next couple of days. Since I'm new to this I do have a couple of questions:

1. Do I replicate the ops table and argmax example that are provided in the vega-lite site? Or should I create distinct examples? Or may I reuse the ones that are provided in this repo?
2. Should the documentation be more technical oriented or example oriented?

Edit: I think it'd be better to have the table of aggregation functions on the Aggregate Transforms section instead of it being on the Binning and Aggregation section. It seems more fitting and would also have the same structure as in the vega-lite site (vl-aggregate-ops). Would doing this be ok?

[joelostblom]

Thanks for offering to help with this! My vote is as follows:

1. Replicate/reuse the ones from the vega-lite site
2. I like how it is done on the vega-lite page for this example. In general I think it is fine to add some additional explanatory text compared to the vega-lite docs because they are sometimes sparse with the explanations.
3. I'm OK with moving it to the aggregate transform section. I think it is slightly better there as you say, but I don't feel too strongly about it either way.

[betaigeuze]

I think this can be closed since #2655 resolved this