Draft: docs restruct proposal

• Based on MR !566 (merged) , trying some further structuring stuff
  • 👉 Review here
• Top level menu with 5 items:
  • 1x Overview/Tutorial (user/persona oriented)
  • 4x Main service types: Helmholtz ID | Helmholtz Cloud | Helmholtz Software | Further services
    each of them subdivided into:
    • How-to
    • Resources/References
    • (Explanation, if any)