Fix all the broken links in the navbar tree about doc-gen4

[gebner]

I don't think that flattening is a good idea mathlib, we've got ~2000 modules.

[xubaiw]

One approach can be to flatten the module hierarchy, e.g.

• Mathlib
  • Mathlib.Algebra
    • Mathlib.Algebra.Group
      • Mathlib.Algebra.Group.Basic
      • Mathlib.Algebra.Group.Defs
    • Mathlib.Algebra.Ring
      • MAthlib.Algebra.Ring.Basic

into

• Mathlib
  • Mathlib.Algebra.Group.Basic
  • Mathlib.Algebra.Group.Defs
  • MAthlib.Algebra.Ring.Basic

which reflects the actual parent-children relation between modules.

[gebner]

I've also noticed two other regressions:

1. The module names no longer fit in the sidebar (which is an issue now that the full names are shown):
   
2. The navbar used to remember the position when navigating to another page (i.e., on loading it scrolls down to the current file). This doesn't work for me anymore.