Doc: Add Qt for INTEGRITY to Reference page

Documentation page reference-overview.html#platform-reference
is missing Qt for INTEGRITY. Add it to the list.
Update also the linked page title.

Change-Id: Ia25295cfb47a40dfdaa172c81764c6c4caa66d86
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>