Doc: edit the qmake manual

Restructure to make it easier to find information.
Fix Qdoc commands and linking. Edit for grammar
and style.
Remove descriptions of obsolete variables.

Task-number: QTBUG-29167
Change-Id: Id44e7f36bbf21fb98df034210f49025b320be361
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>