Warnings during jupyter-book build

[lesteve]

Some warnings when building locally that are likely real issues:

• ensemble_intuitions_index.md

  /home/local/lesteve/dev/scikit-learn-mooc/jupyter-book/ensemble/ensemble_intuitions_index.md:: WARNING: tableofcontents directive in document with no descendants [etoc.tableofcontents]
  /home/local/lesteve/dev/scikit-learn-mooc/jupyter-book/ensemble/ensemble_intuitions_index.md: WARNING: document isn't included in any toctree
  

• video_pipeline.py

  /home/local/lesteve/dev/scikit-learn-mooc/jupyter-book/python_scripts/video_pipeline.py:30002: WARNING: Non-consecutive header level increase; 1 to 3 [myst.header]
  /home/local/lesteve/dev/scikit-learn-mooc/jupyter-book/python_scripts/video_pipeline.py:100002: WARNING: Non-consecutive header level increase; 1 to 3 [myst.header]
  /home/local/lesteve/dev/scikit-learn-mooc/jupyter-book/python_scripts/video_pipeline.py:190002: WARNING: Non-consecutive header level increase; 1 to 3 [myst.header]
  /home/local/lesteve/dev/scikit-learn-mooc/jupyter-book/python_scripts/video_pipeline.py: WARNING: document isn't included in any toctree