Crossreferences support

(As suggested by @rickeylev) Markdown has primitive support for cross-references: you can linkify a term (e.g. ``[`FooInfo`]``) and it will be linked to a named url reference FooInfo, which can be e.g. provided elsewhere on the page - perhaps automatically appended to the documentation page by Stardoc's markdown processor.

We might want to use this mechanism in the following ways:
* automatically linkifying standard starlark and build language types, globals, etc.;
* allowing a `stardoc` rule to specify a custom crosslinks.md input which will get appended to generated markdown output;
* (stretch goal) perhaps automatically linkifying refernces to other rules found in BCR?

CC @comius

Start by locating Stardoc's Markdown processor and the code that assembles generated documentation pages. Determine how named URL references are currently handled, then define completion around automatic links for standard Starlark and build-language symbols and support for a custom crosslinks.md input; the BCR linking idea is explicitly a stretch goal.

Written by the indexing model from the issue text.