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)

Pages Of Interest:
* [Vec](http://cg.scs.carleton.ca/~abeinges/doc/std/vec/struct.Vec.html) (small path)
* [TreeSet](http://cg.scs.carleton.ca/~abeinges/doc/collections/treemap/struct.TreeSet.html) (long path)
* [std](http://cg.scs.carleton.ca/~abeinges/doc/std/index.html) (for stability dash)

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.

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