doc: on skinny displays, don't show navigation UI by default.

also on skinny displays, show a button that toggles navigation UI [in]visible

Testing Done:
Looked at test-publish
http://pantsbuild.github.io/lahosken-okskinny/
with a few browsers, including my Android's built-in and Chrome on Android.

Reviewed at https://rbcommons.com/s/twitter/r/1440/
Sync of https://github.com/pantsbuild/pants/commit/726358b4a63405c640ee583eb8c9bee3a76a8b84