Missing Native Query documentation for Window Functions

### Description

Window functions have been GA in druid since October 2024, but the [current documentation](https://druid.apache.org/docs/latest/querying/sql-window-functions) only covers druid SQL.

It would be great to extend the documentation to cover the `windowOperator` query type in native queries.

### Motivation

The native query syntax is much friendly for programatically generating queries, we'd like to use window functions but the lack of documentations means we need to figure out the syntax by `EXPLAIN`'ing queries or reading the druid source.

Start with the existing SQL window functions documentation at docs.druid.apache.org/docs/latest/querying/sql-window-functions and compare its coverage with the native windowOperator query type. Use EXPLAIN queries or the Druid source to verify the native syntax, then extend the documentation so users can understand how to generate window-function native queries.

Written by the indexing model from the issue text.