docs: add DataFrame aggregation example (select vs aggregate) #26597

Is your feature request related to a problem or challenge?

Follow-up to #25482 (closes #17874), which allowed aggregate functions in DataFrame::select. That PR added tests but no examples, and the examples were left for a follow-up.

Right now datafusion-examples has no example about aggregation with the DataFrame API. aggregate(vec![], ...) only shows up inside other examples (subqueries, UDAFs), and nothing shows the new select form. A user reading the examples will not find out that select(vec![count(col("a")), sum(col("b"))]) works, or when they still need aggregate.

Describe the solution you'd like

Add a new aggregate example to the dataframe group in datafusion-examples (cargo run --example dataframe -- aggregate) that shows, side by side:

• global aggregation with aggregate(vec![], aggs) and with select(aggs) — same result
  expressions on top of aggregates (for example max(speed) - min(speed)) the old way (aggregate then select with aliases) and the new way (one select)
• grouped aggregation, which still uses aggregate(group_by, aggs), and a note that select does not infer a GROUP BY
• the difference between an aggregate in select (one row) and a window function in select (one row per input row)

Plus generated row in the examples README.

Describe alternatives you've considered

Adding the examples to dataframe/dataframe.rs. That file is already long and covers reading/writing data, so a separate example is easier to find.

Additional context

#25482 — the PR that added aggregate support to select
#21021 — the first attempt, which included a version of these examples

Willingness to contribute

I plan to contribute to this feature independently