sort the generated index file

for qdoc be able to create references to other help files, it is necessary to give it access to the generated index file of the referenced documentation.
as there is apparently no way to extract a usable index from an existing qch file, that alien index needs to be included into the source tree of the referencing documentation, as we do in qtcreator/doc/qt.index.
that file is not only huge as such, but also unsorted, which means that every update is a complete rewrite and thus an utter waste of space in SCM repositories.

please fix the sorting problem short-term, and come up with a proper solution for referencing medium term.