Migrate documentation for Aggregate Functions to code

[jonathanc-n]

Which issue does this PR close?

Closes #12827.

Rationale for this change

Migrate aggregate functions from static docs to new format.

What changes are included in this PR?

Migrated aggregate functions.

Are these changes tested?

Are there any user-facing changes?

[jonathanc-n]

@alamb @Omega359 I wasn't sure how to deal with the regr functions as their file is a little unique and I am not sure how to deal with them. We could possibly deal with them in another pr.

Also might need to look deeper into some definitions and examples for some that didnt have old documentation yet (StringAgg, Grouping, NthValueAgg)

[alamb]

@alamb @Omega359 I wasn't sure how to deal with the regr functions as their file is a little unique and I am not sure how to deal with them. We could possibly deal with them in another pr.

Yes, let's handle them in a follow on PR

Also might need to look deeper into some definitions and examples for some that didnt have old documentation yet (StringAgg, Grouping, NthValueAgg)

I'll try and make a PR for these -- it is great we have found a list of functions that were not previously documented

Update: I see you already did this ❤️

[alamb]

Looks great to me -- thank you @jonathanc-n

[alamb]

I made a PR for the regr functions in #12871

[Omega359]

Nice work! I left a few suggestions for improvements

[Omega359]

Since this was merged already if you want we can create a followup ticket for handling the suggestions.

[jonathanc-n]

@Omega359 Yep sounds good, thanks for the review!