docs: list of aggregation functions supported by ibis

[martin-wiebusch-thg]

Please describe the issue

I can't find a list of supported aggregation functions in the current documentation. The tutorial (https://ibis-project.org/tutorials/getting_started#aggregates) mentions five and says "the list goes on", but where is it?

Code of Conduct

• [X] I agree to follow this project's Code of Conduct

[cpcloud]

Great point! I have trouble finding them myself.

Here's a link to them: https://ibis-project.org/reference/expression-generic#methods-1

We should definitely make this searchable and generally more discoverable.

For now, while we figure out a more generic way to make this work, do you want to submit a PR that adds something like This is an aggregate function. to each of those methods' docstrings?

[IndexSeek]

I'm happy to work on this one!