[UX] Expandable sidebar menu

[bugfolder]

The right sidebar menu of the pages on the docs site is currently displayed in static outline form, showing (mostly) just two levels if you are at the top level. In fact, though, there are pages buried 6 levels deep, e.g.,

Getting Started > Installing > Installation Instructions > Advanced Installation > Protecting against HTTP Host header attacks > Trusted host security setting in Backdrop

Currently, there is no way to even know of the existence of that last page without successively clicking down level-by-level, leaving the page you started on, and wading through a bunch of page loads—all just to find out if there's a page somewhere down in the depths of the menu.

It would be a big improvement to use an accordion-style menu, with indications of which levels can expand, and letting the user click to expand any given level to see what's underneath. And so forth, all without leaving the page the user is currently on. In this way, the user could look for the page addressing their particular interest, then go directly to it.

These sorts of expanding menus are all over the place on the interwebs (they're standard practice on mobile devices), so there's probably an easy existing solution out there.

[bugfolder]

Closing in favor of #220.