How to navigate

[willin]

how about 3 level section folders?

like gitbook config:

- [Chapter 1](c1/index.md)
    - [Section 1](c1/s1/index.md)
        - [SubSection 1](c1/s1/ss1.md)

i want to migrate the gitbook project (https://github.com/js-cool/leader.js.cool) to hugo with material ui

c1/index.md:

date: 2016-08-03T21:07:13+01:00
title: c1
type: index
weight: 60

c1/s1/index.md:

date: 2016-08-03T21:07:13+01:00
title: s1
type: index
menu:
  main:
    parent:  c1
    identifier: c1/s1
    weight: 10

c1/s1/ss1.md:

date: 2016-08-03T21:07:13+01:00
title: ss1
menu:   # How to config this???
  main:
    parent:  s1
    identifier: c1/s1/ss1
    weight: 10