Sketch out cross-references section

[tmcw]

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Refs #4: a proposal for how we could do cross-referencing documentation.

TODO/qualms: this really hinges on how we define 'namepaths' or something similar, but this section does not specify that. Which is probably fine, but links are one of the key places where we should determine if, for instance, you can reference [this.method] and it will automatically resolve to MyClass.method if that's the context of the documentation.