Doc: Edited links JavaScript API page and other documentation fixes.

The JavaScript API page is important to the rest of the documentation
but there was no link to it.

Task-number: QTBUG-40756
Change-Id: Id741a67ac2b57e21da548c2961c622c7516ac8cf
Reviewed-by: Milian Wolff <milian.wolff@kdab.com>