Document class dependency resolution (`@require()` and `@use`, `compile.json`)

[cboulanger]

At the moment, it is not obvious where to look in the documentation if one wants to know how to manually configure the compiler's dependency resolution. We haven't migrated http://archive.qooxdoo.org/current/pages/tool/generator/dependencies_manually.html into the new docs, it seems. But in any case, the information must be placed in a more accessible way and linked from introductory chapters.