auto merge of #16277 : Gankro/rust/responsive-docs, r=cmr
* move some sidebar contents to a title bar when small
* inline description toggle when small
* make out-of-band and in-band content share space, rather than float and clash
* compress wording of out-of-band content to avoid line-wrap as much as possible
## [Live Version Here](http://cg.scs.carleton.ca/~abeinges/doc/index.html)
TBD in a future PR is to convert links in the sidebar into a series of nest ul/li's, so that they can easily be moved to a drop-down in the new title bar. I think this is out of scope for this PR, but am willing to implement it now if desired.