finalize discovery-based UI

It's undocumented since it was never completed; cf. https://github.com/mathjax/MathJax/blob/master/unpacked/config/default.js#L1038 and http://codepen.io/mathjax/pen/FDiEj

Closely related to #939 (closed) (could even replace it?)

• Decide on
  • haze
  • marker
    • switch to hamburger icon? generic math symbol? pilcrow? link-like icon like the headings get in the docs or in github markdown rendering?
  • smoother menu interaction (roll under/over/side, always fixed placement at marker?)
    • e.g., cf bootstrap tooltips
  • optional permanent marker