Admin/website/TODO

renamed gen_list to enum;
added separate;
tuned;

In the mid-time:
- a gentle intro enumeration on Home - "Why Isabelle?"
    * stably running, mature system
    * (something to say about Isar?)
    * big library
    * big community
    * it's free
    * ...
  (to discuss)
- clarify relationship of "overview" and "logics":
    * overview: Isabelle/HOL   
    * logics:   Isabelle system
  (to discuss)