Doc: Add keywords for qmake variable/function reference pages

In the Help mode in Qt Creator, many have accustomed to search e.g.
for 'qmake variable reference' which no longer exists as a title in
the qmake documentation.

This change provides easier access to the qmake reference by
creating keywords for them, making them appear in search results
for searches starting with 'qmake'.

Task-number: QTBUG-32268
Change-Id: If60a0cdc11464a8aeb50c62ddbde9683326e1384
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>