Improve documentation on how to add concept schemes from KOS registry to new jskos-server instance

[stefandesu]

The instructions currently work only for the existing jskos-server and jskos-server-dev instances. When adding a new instance, there are additional steps needed:

• Add the prepare script to the folder (-> this should be centralized because it is needed at multiple points)
• Add a schemes.txt file with the appropriate path to the schemes.ndjson file that the prepare script will generate