Restructure ISC docs

[lucas-tortora]

I'll update the feedback doc with a proposed structure based on the findings, I'll structure it a bit like:

- learn
  > Short description of scope
  - [topic]
    > Short description of scope and expected content
    - [ ] Open question or point to discuss
    - [subtopic or already existing file]
      - [ ] Open question or point to discuss
    ...
  - [topic]
  ...
- build
  ...
- maintain
  ...

So we can have structured approach to our call with Jorge.

[jlvandenhout]

Created the issue here: iotaledger/iota-wiki#1359