Redesign documentation method for multi-module entities (namespaces)

In QTBUG-79052, we try to address the problem of how to document a namespace that contains elements that are documented in different modules. The way this was addressed previously was to create "sub" files for the namespace in each module where its elements appeared and link to all those subfiles in the module where the \namespace command for the namespace appeared.

This did work, prior to the introduction of the comments file change, but now it is causing problems (e.g. QTBUG-79052), and now we have another option for fixing it, namely using sub-comments files. The purpose of this task is to investigate that possibility - after the comments file change is integrated.