Update release documentation for Tails Installer with info about Ubuntu PPA

If we want to distribute Tails Installer through a PPA, we need to update the PPA, each time we upload a new Debian package. This needs to be documented.

Feature Branch: doc/10407+ubuntuppa

Parent Task: #10664 (closed)

Original created by @u on 10414 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information