UBY: optimize for responsive xs (noresponsiverelbar, prev, next, index, ^top^, #sidebar-wrapper)

Default: `noresponsiverelbar=false`

set `noresponsiverelbar=true` in `conf.py` to show full-width rellinks:

* htmltitle
* prev
* next
* index
* ^top^ (to get to the TOC)

closes #12.

In another issue, it might also be nice to put `next` first.