[SNOW-1826257]: Refactor docs to provide one place for supported aggregation functions

[sfc-gh-rdurrani]

1. Which Jira issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Fixes SNOW-1826257

2. Fill out the following pre-review checklist:

   • I am adding a new automated test(s) to verify correctness of my new code
     • If this test skips Local Testing mode, I'm requesting review from @snowflakedb/local-testing
   • I am adding new logging messages
   • I am adding a new telemetry message
   • I am adding new credentials
   • I am adding a new dependency
   • If this is a new feature/behavior, I'm adding the Local Testing parity changes.
   • I acknowledge that I have ensured my changes to be thread-safe. Follow the link for more information: Thread-safe Developer Guidelines

3. Please describe how your code solves the related issue.

There are four functions that provide aggregation - DataFrame.agg, Series.agg, DataFrameGroupBy.agg, and SeriesGroupBy.agg. They share similar code paths, so whenever an aggregation function is added, it generally supports more than one of the APIs. We document each API in a different page though, so code authors need to update 3-4 different docs - which can lead to inconsistent docs (in the case that someone forgets to update one or all of the docs). This refactor moves all documentation of supported aggregation functions to one page, which should help keep the docs consistent and correct.