Sidebar of the document page should not be automatically collapsed

[Glavo]

Now that we have clicked on any of the links in the sidebar, the sidebar will be collapsed and this behavior will seriously affect our reading of the document.

[Blaisorblade]

It's not even consistent. That happens for http://dotty.epfl.ch/docs/reference/intersection-types.html doesn't happen for http://dotty.epfl.ch/docs/reference/enums/adts.html, and it seems that's simply because one is in reference/enums while the other is just in reference. Maybe moving files into subfolders would fix this? PRs welcome.