Major sidebars missing on all pages except API docs

[dcamron]

What can be better?

On the docs>=v1.6.0 the left sidebars are missing on all pages except the API docs. Not sure if a version change or if I broke an override somewhere in #3281. I will check this week.

Importantly, this totally removes access to subsections in User Guide, Dev Guide, etc. that aren't otherwise directly linked.

[dopplershift]

I created a 1.6.x branch here and pushed a cherry-picked version of #3366. Docs for 1.6 correctly redeployed and fixed the issue.