Update documentation about mirroring to use the Arch Github

Currently mirroring to Github shows svenstaro/jelly/basically the person who set up the mirroring as "svenstaro pushed to archlinux/infrastructure", while we want it to show "archlinux-github pushed to archlinux/svntogit-packages"

• Change deploy keys for projects set up by @svenstaro or @jelle
• Update documentation to setup mirroring with the Arch Github account