Make Debian (and derivatives) section more accurate

[sten0]

• Include the conventional copy & paste installation instructions that most Ubuntu (and derivatives) users expect.

• Request that users test their operating system's app store, because a potential Appstream bug would be highly visible and would negatively affect initial user impressions. Such a hypothetical bug should ideally be found and fixed before Ubuntu 22.10's freeze, and certainly before Debian 12's.

• Mention the backports repository, because otherwise users will manually install packages for sid/unstable/testing/bookworm/12 onto stable/bullseye/11. These packages will be nonfunctional, will malfunction, or will break the other package's dependencies.

• Remove link to Developer Information Tracker, because it may also mislead users into creating FrankenDebian, and because it is not supposed to be a user-facing page.

[sten0]

Martchus @.***> writes:

Merged #153 into master.

Thanks!