[docs] do not tell users to delete /usr/bin/python3

Section 3.2.1 of the QEMU (Ubuntu) quick start instructs the reader to
run "sudo rm -rf /usr/bin/python3" and re-create the symlink by hand.
On Debian and Ubuntu, apt and much of the system tooling are Python
programs resolved through that path, so following the guide leaves the
machine unable to install packages at all - including the packages
needed to undo it. The hardcoded 3.9 version in the example also goes
stale with every release.

Replace it with python-is-python3, which is what provides the python
command on these distributions, and add a warning against the manual
approach. Drop the now unreferenced screenshot.

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