Verification docs outdated

[achow101]

The verification documentation is outdated. It says to retrieve keys using contrib/builder-keys/keys.txt, however this file has been removed in favor of having the keys directly in the guix.sigs repo now. The verification docs need to be updated to reflect this.

[fanquake]

See #807, #894, #878 #891 etc

[maflcko]

Looks like #878 (comment) is up for grabs

[samylaumonier]

In the meantime, PGP fingerprints are available here

[pinheadmz]

See #807, #894, #878 #891 etc

All incorporated into incorporated #964 now