[Shock] - Automatic linking to API symbols

[matthew-carroll]

Documentation often includes references to source code classes. There should be an easy way to write documentation such that those references are found, their corresponding pub doc URL is discovered, and the reference is linkified in the final HTML output.

Consider that packages have different release versions, and each release version has its own docs. There's also a "latest" docs URL, which corresponds to the latest regular release (not preview releases).