v0.20.1 — Surface model: docs & showcase

A documentation-and-showcase patch on top of v0.20.0. The surface model shipped with per-rune reference docs; this milestone makes it land — a cohesive showcase of the vocabulary as a system — and finishes the deferred composition/showcase work from v0.19.0, now that both the bento substrate and the surface model exist to feed it. No new runtime features.