Write up scan server example for Blog post

**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

Start by reading the scan-server example in server/tserver/src/main/java/org/apache/accumulo/tserver/ScanServer.java around line 130 and ConfigurableScanServerSelector.java around line 150. Use those entry points to explain scan servers and the ScanSelector in a blog post, then publish the completed post.

Written by the indexing model from the issue text.