Provide a directive to link the documentation on the registry

[schveiguy]

The dub registry currently has a "setting" that you must add at the registry in order to add a documentation link.

Following the precedent of the "homepage" and "description" directives, neither of which factor into building but are exclusively for the registry, I propose the dub file add a new directive "onlineDocumentation" which consists of a URL to use for documenting a package. This way, we don't need that extra setting, and running e.g. a CI script to build the appropriate docs, deploy, and update the json file is all doable without dealing with APIs of dub registry.

In fact, I would like to see all the configuration of the registry package (logo, categories, etc.) aside from the fundamental permissions and repository link be put into the dub file. But for now I think the documentation is the most obvious one.