Stardoc should support linking to other symbols

... a la javadoc's `{@link ...}` construct. The syntax of the link should not depend on the output format (`` vs `[...](...)`), and should not require embedding any URLs but rather directly use names of imported symbols.

(Unimported symbols might be referenced via the .bzl label that would've been used had they been imported, so you don't have to load things just for the sake of documentation completeness. But I'm not sure how that interacts with searching for transitive dependencies, bzl_library, etc.)

The issue names no files or tests. Start by determining how Stardoc represents imported symbols and emits links for different output formats, then resolve how unimported .bzl labels, transitive dependencies, and bzl_library should interact. Done means a format-independent link syntax is defined and its behavior is specified for these cases.

Written by the indexing model from the issue text.