Support for nested pages

[tiefenthaler]

Mkdocs-jupyter does not support nested pages to be configured in the mkdocs.yml like:

nav:
  - Home: index.md
  - Source Code API Reference:
      - Autoencoder: api/autoencoder.md
      - Training: api/training.md
      - FastAPI: api/fastapi_app.md
  # NOTE: Does not support nested lists.
  - Notebooks 
    - Main: main.ipynb
    - Demo: demo.ipynb

Only supports not nested structure:

nav:
  - Home: index.md
  - Source Code API Reference:
      - Autoencoder: api/autoencoder.md
      - Training: api/training.md
      - FastAPI: api/fastapi_app.md
  # NOTE: Does not support nested lists.
  - Notebooks - Main: main.ipynb
  - Notebooks - Demo: demo.ipynb