Comment 163 for bug 114441

I checked out Ruby documentation, and it turns out we were doin g pretty well; we were just missing a page on the <rb> element. I've added this now, along with associated compat data and an interactive example:

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/rb
https://github.com/mdn/browser-compat-data/pull/2422
https://github.com/mdn/interactive-examples/pull/1016