Preview navigation children in left bar

[oskarth]

Problem

Currently you have to be on a child node in ToC to see siblings:

These child nodes should be visible from main page, but currently they are not:

Notes

Probably a quick theme fix / option with https://github.com/pmarsceill/just-the-docs

I had some issues getting the whole Jekyll shebang running in a timebox, so just doing live edits right now. This change would benefit from tighter feedback loop by running locally with SASS or whatever setup is needed.