Share prose / markdown between languages?

[alexcjohnson]

There are lots of places we either have the same or very similar text in the Py and R docs, or we don't and one (mostly R) is behind. Noticing the latter situation in #928 - it's particularly evident there since we're talking about the renderer, and very little needs to change between the two versions.

If we had a way to share this prose, it would help us keep the two (and hopefully soon three https://github.com/plotly/dash.jl) flavors consistent. Also potentially something to think about re: localization #891