Add search tab (same level as glossary) for easier searching through the course

Something which I find is missing is searching through the course, see item 4. in https://github.com/INRIA/scikit-learn-mooc/issues/344.

An improvement could be to add a search tab (same level as glossary). It could be an iframe with this as content:
https://inria.github.io/scikit-learn-mooc/search.html?content_only

So that a user could have the search fonctionality from JupyterBook while still staying inside FUN. To give an idea how this would look:
https://inria.github.io/scikit-learn-mooc/search.html?q=OrdinalEncoder&content_only

This search tab could also be a place to remind people that they can browse the material on our .github.io, since this is slightly more convenient that clicking around in FUN.

Caveat: the state of the `.github.io` is not completely is not in sync with the user Jupyter notebooks, but I think that's fine ...