Remove ETL and document alternative  #2959

The ETL project is no longer something we want to spend time thinking about and are planning to drop it (move it to another repo for where it will hopefully fade into obsolescence). ETL being one of the most common and most important operations when working with large amounts of data and rasters almost always being large amounts of data, we'll need to provide alternative mechanisms and updated documentation that guides users towards writing code that is a bit less sui generis.

These tasks need to happen for us to tear out the ETL module:

• Review documentation for ETL and update it with direction to ETL's new home (gt-contrib)
• Surface pipeline documentation as an alternative (it is currently not advertised in the docs very well)
• Document how RasterSources can make life easier and provide example code
• ETL archived elsewhere