cross module API docs

[tombentley]

This came up on IRC just now... Although there's a page for the modules in the SDK (https://modules.ceylon-lang.org/categories/SDK) that's less helpful than it could be when it comes to browsing the docs of the SDK "as a whole" -- I have to navigate to each module and then pick the version whose docs I want to view. Then when I get to a particular API doc it doesn't link to the declarations of another module within the SDK. Heck, they don't even link to the declarations from the language module!

Put these together and we have something which just isn't browseable, and is therefore a barrier to people learning these APIs.