WIP: Add new documentation about what jobs run on what releases

So far Salsa CI has been focused on testing packages prior to upload to Debian unstable (sid), while testing packages prior to upload to other releases has been a secondary feature. The purpose of this documentation is to establish what jobs run, why and where so we have a shared understanding of how the pipeline is supposed to morph based on the RELEASE value. With this documentation established, it is easier to decide and implement each individual job and its behavior in different Debian releases, and even other distros (currently Kali and Ubuntu).

For rendered Markdown see https://salsa.debian.org/salsa-ci-team/pipeline/-/blob/f69d4b29aae628160ff0a95e8e9550dade06079c/JOBS-PER-RELEASE.md