Doc: Improved Qt Quick Demos documentation

-added include from qtbase/doc/global about running the example.
-added link to "QML Applications" (main landing page for QML)
-highlighted the names with \e
-various documentation changes

Task-number: QTBUG-33597
Change-Id: I9e23b8b61862c424f2a17003b3d1f5b2ef931ceb
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>