docs/distributions: rearrange, add centos
docs/distributions: rearrange, add centos

joshix/rkt,shobhit85/rkt,alban/rkt,squeed/rkt,Shashwatsh/rkt,jakon89/rkt,sstabellini/rkt,ybubnov/rkt,nhlfr/rkt,squeed/rkt,rkt/rkt,ybubnov/rkt,shobhit85/rkt,coreos/rkt,kinvolk/rkt,joshix/rkt,coreos/rkt,ybubnov/rkt,jonboulle/rkt,jonboulle/rkt,lucasallan/rkt,rkt/rkt,sstabellini/rkt,achanda/rkt,lucasallan/rkt,rkt/rkt,nhlfr/rkt,lucasallan/rkt,kinvolk/rkt,alban/rkt,cab105/rkt,achanda/rkt,intelsdi-x/rkt,rkt/rkt,nhlfr/rkt,kinvolk/rkt,cab105/rkt,intelsdi-x/rkt,jonboulle/rkt,Shashwatsh/rkt,jakon89/rkt,dgonyeo/rkt,jakon89/rkt,intelsdi-x/rkt,shobhit85/rkt,lucasallan/rkt,cab105/rkt,kinvolk/rkt,sstabellini/rkt,achanda/rkt,squeed/rkt,joshix/rkt,dgonyeo/rkt,alban/rkt,coreos/rkt,dgonyeo/rkt,joshix/rkt,Shashwatsh/rkt