Add native support for cross-referencing rules and rule attributes

While building out a set of rules, I've been mentioning various rules and rule attributes in doc strings, and wasn't able to find any portable way to do cross referencing between generated docs. Since I've been exporting to markdown, I've been solving this via `[foo](#bar_rule-foo)` entries to cross-reference anchors on the same page. Unfortunately, this syntax is not inherently portable to other doc generation schemes, so if I regenerate the docs to a new format (which I'd like to do eventually) there's no guarantee that the links will continue to work.

It would be really great if stardoc could be extended to natively support expressing cross-references in document generation in a portable way.

No source files, tests, or entry points are named; begin by locating the document-generation path and its format-specific link handling. Done means a documented portable cross-reference mechanism that remains valid when documentation is regenerated in different formats.

Written by the indexing model from the issue text.