[docs]Add some working examples for functions in aggregate.rst

[KIRITOLTR]

Description

Adding some simple examples for functions in aggregate.rst.

Motivation and Context

For addressing the lack of working examples in functions docs. See #19830

Impact

Documentation.

Test Plan

Local doc builds. Some Screeshot:

Contributor checklist

• Please make sure your submission complies with our contributing guide, in particular code style and commit standards.
• PR description addresses the issue accurately and concisely. If the change is non-trivial, a GitHub Issue is referenced.
• Documented new properties (with its default value), SQL syntax, functions, or other functionality.
• If release notes are required, they follow the release notes guidelines.
• Adequate tests were added if applicable.
• CI passed.

== NO RELEASE NOTE ==

[steveburnett]

LGTM! (docs)

Pull branch, local doc build, looks good.

Thanks for all of these examples!

[steveburnett]

Checked the examples and found one that might need some work. The others seem good.

[steveburnett]

This example when I tested it returns the following error:

Query 20250527_155203_00071_afft5 failed: line 1:20: Unexpected parameters (integer) for function khyperloglog_agg. Expected: khyperloglog_agg(E, T) E, T
SELECT cardinality(khyperloglog_agg(user_id)) AS approx_distinct_users
        FROM (
        VALUES (101), (102), (103), (101), (104)
        ) AS t(user_id)

Can you take a look and maybe fix this one, or delete this example if fixing it is complicated?