A systematic way to customize TOC in layers is needed

[strongoier]

While the current TOC can be written arbitrarily in a manual way (which is what we want), it is lacking layers, which is a bit non-intuitive:

taichi: The main taichi module
taichi.ad: Taichi Autodiff system
taichi.aot: Taichi AOT system
taichi.linalg: Taichi Linear algebra library
taichi.profiler: Taichi profiler tools
taichi.tools: Miscellaneous tools shipped with Taichi
taichi.types: Taichi types
taichi.types.quant: Taichi quantized types
taichi.ui: Taichi UI components (GUI and GGUI)

We hope the TOC can present layer relationship like the following:

taichi: The main taichi module
  taichi.ad: Taichi Autodiff system
  taichi.aot: Taichi AOT system
  taichi.linalg: Taichi Linear algebra library
  taichi.profiler: Taichi profiler tools
  taichi.tools: Miscellaneous tools shipped with Taichi
  taichi.types: Taichi types
    taichi.types.quant: Taichi quantized types
  taichi.ui: Taichi UI components (GUI and GGUI)