Add documentation example for `AggregateExprBuilder`

[alamb]

          This looks very similar to https://docs.rs/datafusion/latest/datafusion/physical_expr/aggregate/struct.AggregateExprBuilder.html#method.distinct

Though it seems like that structure has no good example, perhaps you could make a PR to add an doc example of how to use it

Originally posted by @alamb in #14413 (comment)

The idea is to add some examples in the rust API documentation showing how to create expressions like

first_value(a ORDER BY x)

Using the AggregateExprBuilder

Something like this (not quite the actual syntax)

let agg_expr = AggregateExprBuilder::new(first_value_udf())
  .args(col("a")
  .order_by(..)
  .build()?

[Shreyaskr1409]

take

[Shreyaskr1409]

@alamb should I add this example to datafusion-examples as well? as per #15504 (comment)