Adapt packaging process and release doc for Tails Installer DFSG-free tarball

These docs live in contribute/release_process/liveusb-creator and contribute/release_process/liveusb-creator/topic_branch.

Feature Branch: doc/8555-update-installer-release-doc

Original created by @intrigeri on 8555 (Redmine)

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