docs: update release guide with signing details
docs: update release guide with signing details

This commit updates the release guide with detailed steps for
signing tags and release artifacts in a consistent way.

Fixes #2734

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