[Website] Aggregating Millions of Groups Fast in Apache Arrow DataFusion 28.0.0 (#386)

Closes apache/datafusion#6988

**Note**: This describes work @tustvold @Dandandan and I did in
DataFusion 28.0.0. This content was originally published on the
[InfluxData
Blog](https://www.influxdata.com/blog/aggregating-millions-groups-fast-apache-arrow-datafusion/)
but since it is general applicable to Apache Arrow DataFusion I would
like to syndicate it here becase:
1. This is a form where the community can comment / keep it up to date
via PR
2. It is hosted on a platform with a different lifetime than a company
blog

This is the same model we followed with
https://arrow.apache.org/blog/2022/12/26/querying-parquet-with-millisecond-latency/
which was also republished on the arrow blog after the InfluxData blog

It also gives me an example to use my original ASCII art diagrams :)

Author: alamb