Add 'Learn & Discuss' section/toplink to www.coucdhb.org page - COUCHDB-1863

- The new section encompasses documentation and the old Mailing Lists section.
- "Learn & Discuss" is the most descriptive short name I could think of
  for "Documentation and Mailing Lists"
- Mailing-Lists internal anchors kept for backwards compatibility
- Staged version at http://hiperactivo.com/~kandinski/couchdb.org/