Write up scan server example for Blog post

[ddanielr]

Is your feature request related to a problem? Please describe.
We currently don't have great documentation for using scan servers.

Describe the solution you'd like
A blog post should be written up and published for using scan servers and the ScanSelector.

https://github.com/apache/accumulo/blob/2.1/server/tserver/src/main/java/org/apache/accumulo/tserver/ScanServer.java#L130

https://github.com/apache/accumulo/blob/main/core/src/main/java/org/apache/accumulo/core/spi/scan/ConfigurableScanServerSelector.java#L150

[dlmarion]

I tried to keep this comment (#2665 (comment)) up to date during development and testing. It could be used as the foundation for a blog post.