docs: Early boot debugging over serial (rd.break) — guidance and kernel cmdline in install-debugging-tools.md

[tbringuier]

Add a concise, provider-aware recipe to debug early boot over the serial console. While debugging Flatcar on Scaleway, several systemd units failed without visible errors on the default console; this PR adds a short section to install-debugging-tools.md with a canonical kernel cmdline to force an interactive rd.break on serial only and increase verbosity:
rd.break rd.shell rd.debug loglevel=7 console=ttyS0,115200n8 earlycon=ttyS0,115200 systemd.show_status=yes systemd.log_level=debug systemd.log_target=console
It also notes that the last console= gets input and that providers like Scaleway require explicitly enabling and targeting the correct serial device.

[tbringuier]

Thanks, we had issues with Scaleway but submitted a PR to fix the serial output directly through the OEM flavour.
So it is no more needed to precise whether the serial port or baud in the dracut arguments since the correctly and updated flavour used should not have that type of issues.

I think it should be interesting to inform users that rd.earlytrace and rd.earlyshell exists.

It is no more needed to precise "Scaleway" provider, and we should just add it as a generic debug method inside the documentation.

[tormath1]

Thanks for this PR. FWIW, there's some similar documentation available here: https://github.com/flatcar/flatcar-website/blob/main/content/docs/latest/provisioning/ignition/boot-process.md#debugging-ignition
Maybe we should cross-reference them or merge them.

[tbringuier]

Indeed, I think it would be appropriate to move what already exists to the debugging section and combine my proposal with what is already there in order to make it clearer and more user-friendly.

[t-lo]

We can also just merge and refactor later; we've been planning a larger restructuring anyway. @tormath1 wdyt?