docs/PORTABILITY_AND_STABILITY: update the policy

docs/PORTABILITY_AND_STABILITY: update the policy

The kernel has a "don't break userspace" policy [1] which is very easy
to understand and like (even if the details are complicated). We had a
complicated policy statement, but the actual practice is that we'll
generally avoid changes that cause problems for users. So the whole
detailed table is more of a guideline than something that we can rely
on.

To improve public perception, and to align the docs with practice, let's
make a general promise to keep stability, and change the existing table
to a "guideline" that tells people which interfaces to use and which not
to use so that use and development of systemd don't conflict or create
pain.

The existing document was very long, and I think it'll have a bigger
impact if it shorter. In particular, the chart is dropped, because it
was of limited use and severly outdated, because we have added many new
interfaces and components without updating it. Adding the missing
elements (and then keeping it updated) would be a lot of effort. The
chart would be most useful to somebody trying to build a systemd
reimplementation, but there haven't been many attempts like this, so
let's save the work and drop it and make the document shorter.

I also dropped the mentions of Debian. Back in the day, we did indeed
adopt many Debian conventions for config files and such, but over the
years the scope of the project has grown quite a bit and that part
of systemd is a small fraction now. And we've adopted conventions
and ideas from many other places too, so we don't mention Debian
specifically.

[1] https://docs.kernel.org/admin-guide/reporting-regressions.html