Crossreferences support

[tetromino]

(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