Improve docs on Aggregate Transforms #2645

[tempdata73]

So this is what I've worked on with respect to #2645. A couple of notes:

• I messed up my commits and branches, so the first two are just noise and won't affect the repo in any way whatsoever.
• Similar to the previous point, I merged the add-argmax-example branch with this one, and so it appears that I want to add an example that's already been added in another pull request I made. My guess: it's because that hasn't been merged yet.
• I didn't use vl's example of using argmax on the movies dataset because this whole page uses the cars dataset as a guide. I felt like using the former would break the flow of thought. Nonetheless, I can happily incorporate it if you guys prefer that one.

I hope this is what you're looking for, but if it isn't please tell me and I'll try to fix it right away.

[joelostblom]

@tempdata73 I think the changes you suggested here look good and I am not sure why the PR was closed. I can reopen it but maybe you want to create a new one based on the latest main branch and remove the unnecessary commits you mentioned?