Not sure when this happened but i love the new left-hand side navigation https://scikit-learn.org/dev/user_guide.html

(@adrinjalali did this maybe?)

However, when clicking the different entries, the result is inconsistent. For some, it shows a TOC that expands the existing toc with more detail:

For others it replaces the TOC entirely:

I prefer the first one and I think we should adjust the TOC on all of the pages so that the high-level overview stays present on all of the pages.

That requires changes of getting started, glossary, api reference and examples.