Thanks I’m interested too so I’ll check the theme.
I made a repo which let’s me list content any number of levels down and only the current folders and pages are shown as a menu so nesting doesn’t get crazy. There’s no sidebar though but maybe the dynamic menu that I’ve made can be turned into a sidebar this also dynamic if that suits you.
My proof of concept before that: https://github.com/michaelcurrin/nested-jekyll-menus
I am a Jekyll fan but for interest, here are some non-Jekyll solutions:
MkDocs is lighter on code I feel than Jekyll and provides much easier theming and sidebar and even search!
Without knowing ins and of each theme. You can make it look like Read The Docs specifically. It supports sidebar nesting up to 2 levels only and set by config YAML
See my quickstart
Also check out Docsify. Theming is mostly about changing colors rather than layout but you get a nested sidebar up to many levels from YAML. You can also configure how many levels should be shown. Another nice thing is that the subitems in the sidebar get compressed when you’re not on that section so it keeps it sane.
Plus you can have multiple menu files to determine different sidebars. Then you use the header nav to get to sections of the site and each section has a sidebar of relevant content. Which means less nesting in the sidebar.