Add documentation for Debian repository

[ann0see]

Related to: jamulussoftware/jamulus#3013

Once the GPG key is set up on the main repository, the CI should automatically generate a debian repo on releases. This should be documented on the Linux installation page. Probably even as default installation method.
After we've pushed a nightly, I think we can start writing a paragraph.

[pljones]

Can this now move back to "In Progress"?

[ann0see]

Yes. I'm working on getting the repo stuff fixed. It seems to fail on importing the key even though it seems to do something.