Feature Request: Hierarchical File System and its export

[EazyReal]

Hackmd is an amazing project.
However, I think it will be better with the support of a hierarchical file system.

Decription of Feature Request

It seems that the current notes are saved sequentially and displayed accordingly to users.
For example:

• hackmd-user1
  • foo.md
  • bar.md

It would be great to see hierarchy support like:

• hackmd-user1
  • lecture_notes
    • linear_algebra.md
  • life
    • must-go_resturants

Potential Solution

It is plausible to handle this on one's own by providing "parent" in each header yml.
But I think it would be great to have this feature built in HackMD.

[jesseduffield]

This is the killer feature that's missing for my team: we want to use a markdown-first docs tool but we need the hierarchical doc structure