qdoc: Allow tying a \keyword to a specific page (sub)section

QDoc's \keyword command creates a keyword that acts as a link target for the topic the command appears in. In addition, the keyword is added to the .qhp, making it appear in the Qt help index, and available for the context help in Qt Creator.

However, because the keyword is always tied to the topic itself, it's impossible to refer to a specific location within the topic (e.g. \page). Use case; add keywords to the index, where each kw refers to a section title in the documentation.