generated docs: source listings with links back to docs

[andrewrk]

Extracted from #21

The idea here is you can easily get to the source for a given function, decl, or type, and while looking at the source, get back to the docs. Functions would list callsites so that you could look at an example or at least a test of how something is used.

[Vexu]

Added to Autodoc in #12374