Improve the UX of the sidebar

[b-fuze]

The sidebar on the previous design was super helpful. While reading the docs of any specific item, I would usually quickly scroll the sidebar for something else if I didn't find what I was looking for. Nice and clean new design btw.

[GRoguelon]

If I can add something, I'm new on Deno and having the list gives a pretty good idea of the methods available.

[crowlKats]

How about all "categories" (ie Namspaces, Classes, etc) in the sidebar can be expanded with a chevron and then a list of all the items for that category are listed there?

[joshyrobot]

An expandable list could work, but not with the categories themselves being links too. I would very much prefer an expandable list of links over the current situation, though.