There are many sources of documentation available for using hamradio software in Debian. All software packages will have man pages included and many also include HTML or PDF-formatted documentation too.
A comprehensive guide for using amateur radio software in Debian is currently in development but is not yet available. If you would like to contribute to this effort, please see the related wiki page.
According to Debian policy, every application in Debian must contain a man page and these can be very informative. They are typically more useful for command-line driven applications although can also provide useful information for GUI applications. To view the man page for an installed application, run:
$ man <application>
If you'd like to view the man page for an application that you do not currently have installed, you can search at manpages.debian.org.