man: add libsystemd(3)

man: add libsystemd(3)

Before libsystemd-daemon, libsystemd-journal, libsystemd-id128, etc., were
merged into libsystemd, it was enough to have individual man pages for them.
But they have been delivered as one thing for many years, so it's better to
have a landing page for libsystemd. It mostly directs to individual pages
anyway.