docs/site-example: cleanup

 - generalize, to make it more clear that the site config
   needs to be adapted to the local community.
 - remove some host names and public keys used in Lübeck
 - suggest private space mac adresses
 - change fastd method to salsa2012+umac, which is the recommended default
 - add config for a fastd peer reachable by both ipv4 and ipv6
 - order GLUON_SITE_PACKAGES alphabetically