docs: add Debian/Ubuntu installation via deb.griffo.io (unofficial apt repo)

[dariogriffo]

I maintain deb.griffo.io, an unofficial apt repository for Debian and Ubuntu whose packages are built automatically from the official jj releases within hours (packaging is open source at https://github.com/dariogriffo/jujutsu-debian). This adds a Debian/Ubuntu subsection to the Linux install docs, clearly labelled as unofficial, alongside the existing unofficial COPR entry for Fedora.
The repo offers an annual subscription for the automatic updates, yet the packages can be freely downloaded from the GitHub repos where they are built

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[pksunkara]

While I am okay with this for now, I would like to raise the question on whether we want to start maintaining our own package repositories.

Ideally, having this packaged by a distro is better, but I am unsure what that takes.

[PhilipMetzger]

I would like to raise the question on whether we want to start maintaining our own package repositories.

I still don't want that since we're not anywhere near stable.

[joshka]

I've put together a working alternative at https://apt.joshka.net/ and opened #9837 to move the project-ownership question into a dedicated issue.

The implementation is at https://github.com/joshka/apt.joshka.net. It packages the official static release artifacts, publishes only new package payloads, retains older versions for pinning, and is designed to be handed over to project-owned infrastructure and signing keys.