NEWS: reorder entries with some minor additions

Let's make this easier for readers by grouping common subjects together.

Roughly: pid1 features, unit file changes, general syntax changes, kernel
options, general defaults, udevd features, networkd and .network/.netdev
features, networkctl, resolved, systemctl, systemd-run, journald, journalctl,
various other tools, low-level dbus and library stuff, documentation.