qdoc: Never use a collision page as a main QCH landing page

When writing the main/landing page for a documentation set to
a .qhp file, check that it's not a collision page. If it is,
default to 'index.html' instead.

Task-number: QTCREATORBUG-11035
Change-Id: I0269834f6e50fcf5ae779c026263e69e4c3cd7b6
Reviewed-by: Martin Smith <martin.smith@digia.com>