The design and overall concept of the sidebar looks lovely. I explored it with @ethankusters temporary overlay of ArgumentParser.
Seeing the filtering, I immediately reached for it - and was a little surprised when content was highlighted by the filtering action. Then it was even more awkward as some of the titles that matched the filter value I used weren't easily visible in the sidebar, sort of highlighting the difficulty of the "too much horizontal space used by the sidebar" while also "not enough to be visible". (The article titles were frequently cut off)
I also think that using a tag (for example, to just show an article or tutorial) is completely opaque and if you hadn't called it out in your introduction, I wouldn't have had a clue that was an option. In fact, I'm not even sure today how to trigger the filter to just show articles within the module.
I think the filtering mechanism is important, and even better (and a much more involved implementation) would be a full fuzzy search that returned based on some amount of potential relevance. For either of these cases, I'd like to advocate for the possibility of having a separate result area displayed that shows the results, and leaving the side-bar and it's navigational structure untouched.
While I love the additional detail being available for the functions in the sidebar, as others have pointed out, it's pretty heavy space-wise, and might suggest that the most valuable thing to display is the symbol's name, followed by arguments. I do like the box to the left indicating the kind of symbol (protocol, struct, enumeration, class, etc), but it's the thing that allows me to disambiguate between two otherwise similar symbols. The cases where I've wanted or needed to that seemed to boil down to the arguments that a function or method took - and often the type associated with the argument - which is probably more than can easily fit in the sidebar.
I don't know who enabled the option-click-disclosure to fully expand a tree downward, but THANK YOU!!!! That's hugely beneficial, and far easier for me to navigate than the scroll-down-click-scroll-down-click sequences.