Fix remaining issues for 0.6.1

[apyrgio]

Fix various issues that remain for 0.6.1:

• Add a script for hashing and signing our release assets
• Add end-user instructions about verifying these signatures
• Add a release note for installing the latest Python from Python.org on macOS
• Add some information in our docs about extra packages that will be installed in some user systems from our Linux repos (python3-pyside6 / conmon)

Refs #471 Fixes #767 Closes #761

[!NOTE] In order to visualize the changes for the verification instructions, reviewers can take into account the following:

1. I have updated our v0.6.0 release page with signatures and checksums.
2. I have sent a PR for our website (https://github.com/freedomofpress/dangerzone.rocks/pull/37), that shows how we can point to our verification instructions.