Render docs (and tests?) linked to definitions

[nicklawls]

The idea here is simple, just pull up everything linked to a definition and render it pretty.

But, while rendering docs, what should we do about the builtin.Doc.Evaluate constructor? Maybe just render a placeholder for now, and in the future whip up a server endpoint that can evaluate definitions on the fly. Unless someone wants to implement a Unison interpreter in Elm! 😈