Add captions to sidebar

[Mathieu-COSYNS-Student]

This PR add captions to the sidebar to better distinguish different parts of the book.

Drawback: In order to maintain the current style of headings in index.html and the structure of the sections in the pdf titles are duplicated.

##################
Part 1: Principles                      <---------------- HERE
##################

.. toctree::
   :maxdepth: 4
   :caption: Part 1: Principles         <---------------- HERE

Why adding a few lines of css ? The duplication add the title twice in index.html as shown below. The css fix the issue for html, singlehtml and epub. Captions are not visible in pdf