Documentation/packaging: Name and explain gc and metadata-svc unit files
Documentation/packaging: Name and explain gc and metadata-svc unit files

Make a sensible headings hierarchy.
Order sections sensibly: build concerns, run concerns.
Note the included systemd units: gc, metadata-service.
Fixes #1636.

squeed/rkt,alban/rkt,rkt/rkt,rkt/rkt,rkt/rkt,sgotti/rkt,matthaias/rkt,rkt/rkt,Wilybald/rkt,cab105/rkt,blixtra/rkt,mike1729/rkt,intelsdi-x/rkt,euank/rkt,kinvolk/rkt,blixtra/rkt,lucasallan/rkt,coreos/rkt,Wilybald/rkt,sstabellini/rkt,jjlakis/rkt,euank/rkt,endocode/rkt,yifan-gu/rkt,joshix/rkt,yifan-gu/rocket,euank/rkt,tmrts/rkt,dgonyeo/rkt,tmrts/rkt,ybubnov/rkt,jjlakis/rkt,sstabellini/rkt,squeed/rkt,Wilybald/rkt,kinvolk/rkt,mike1729/rkt,endocode/rkt,nhlfr/rkt,sgotti/rkt,s-urbaniak/rkt,markllama/rkt,lucasallan/rkt,jakon89/rkt,mike1729/rkt,dgonyeo/rkt,joshix/rkt,sinfomicien/rkt,achanda/rkt,sinfomicien/rkt,squeed/rkt,monstermunchkin/rkt,yifan-gu/rkt,markllama/rkt,jakon89/rkt,Shashwatsh/rkt,ybubnov/rkt,matthaias/rkt,yifan-gu/rocket,ybubnov/rkt,sinfomicien/rkt,jonboulle/rkt,monstermunchkin/rkt,blixtra/rkt,joshix/rkt,yifan-gu/rocket,sstabellini/rkt,jjlakis/rkt,achanda/rkt,tmrts/rkt,alban/rkt,monstermunchkin/rkt,intelsdi-x/rkt,s-urbaniak/rkt,kinvolk/rkt,nhlfr/rkt,il9ue/rkt,lucasallan/rkt,akshaykarle/rkt,shobhit85/rkt,coreos/rkt,nhlfr/rkt,jonboulle/rkt,akshaykarle/rkt,markllama/rkt,coreos/rkt,achanda/rkt,lucasallan/rkt,jakon89/rkt,endocode/rkt,shobhit85/rkt,matthaias/rkt,shobhit85/rkt,dgonyeo/rkt,akshaykarle/rkt,Shashwatsh/rkt,il9ue/rkt,yifan-gu/rkt,s-urbaniak/rkt,alban/rkt,cab105/rkt,joshix/rkt,Wilybald/rkt,intelsdi-x/rkt,il9ue/rkt,sgotti/rkt,kinvolk/rkt,jonboulle/rkt,cab105/rkt,Shashwatsh/rkt