fix #4847: network: extend section on interface naming scheme

fix #4847: network: extend section on interface naming scheme

Expand the existing section on systemd network interface names with a
link to the systemd.net-naming-scheme(7) manpage and some information
about naming scheme versions. Also mention the possibility of
interface naming changes due to a new naming scheme version, or
kernel/driver updates. This happens for quite some users during the
upgrade from PVE 7 to 8.

Further, describe how to pin a specific naming scheme version and how
to override interface names using systemd.link files.

Also, make some formatting fixes to the existing text.

Signed-off-by: Friedrich Weber <f.weber@proxmox.com>