The system is spoiled by confusion between division into 10 and division into 100. This creates extra levels so that the implementation does not live up to its "two clicks away" promise.

For instance, in the site's own structure, we have

  11-core/11.01-introduction

But that would leave two digit categories at the top level. The top level is organized by groups of ten and so we need

  10-19-concepts/11-core/11.01-introduction

One question is what if 10/11 gets more than ten items, so there is an 10/11/11, 10/11/12?

Isn't there a division into ten needed there?

If the bottom level never goes beyond 00-09, the zero is redundant. It's actually a three level system with a branching factor of 10, and you might as well just have

  10-concepts/11-core/1-introduction

I would just have

  10/11/1

and have symlinks

  concepts -> 10

  10/core -> 11

  10/11/introduction -> 1

Using the numbers as prefixes for the symbolic names means that someone who remembers the symbolic name but not the number cannot use tab completion nicely. They have to use tab completion to scan the entire directory level, then type the number, then tab complete again.

Symlinks going from symbolic to numeric is probably the right direction. The OS symlink resolution then teaches the users what the categories are:

  $ realpath --relative-to=. concepts/core/introduction
  10/11/01

There could be accelerator symlinks at the top level:

  11.1 -> 10/11/1

Now you get the full benefit. If you remember that introduction is 11.1, you actually have that as an instantly navigable identifier in the system.