Add docs for all repositories

• normal ce
• incubation (https://app.camunda.com/nexus/content/repositories/camunda-bpm-incubation/)
• snapshots (https://app.camunda.com/nexus/content/repositories/camunda-bpm-snapshots/) --> this could be a hint on the download page (I do not want people to get mixed up with the real versions!)

and in special area

• EE

Also document Nexus (URL, usage, login for EE, ...)