man/systemd: reorder content a bit

man/systemd: reorder content a bit

Section "Description" didn't actually say what systemd does. And we had a giant
"Concepts" section that actually described units types and other details about
them. So let's move the basic description of functionality to "Description" and
rename the following section to "Units".

The link to the Original Design Document is moved to "See Also", it is of
historical interest mostly at this point.

The only actual change is that when talking about API filesystems, /dev is also
mentioned. (I think /sys+/proc+/dev are the canonical set and should be always
listed on one breath.)