Link to tests as minimal feature examples

[jrk]

We have tutorials and READMEs, but our most complete documentation and reference really only exists in two places:

• Docstrings (and the Doxygen-generated docs)
• Tests (especially all the correctness tests)

The docstrings are useful, but often users need more complete reference on how to use a given feature. Core developers know that correctness test examples exist for effectively everything in the language, but most users don't.

1. Perhaps the generated docs for features should link to a corresponding test program as a minimal example, where possible.
2. Perhaps other documentation that mentions a feature should consider doing so, as well.