Unlike the QTextBrowser backend, litehtml does not render elements
inside <iframe> correctly. This prevented external links to
YouTube from working in offline documentation.
Move the <iframe> to a macro override specific to online doc builds.
Pick-to: 6.0 6.0.0
Fixes: QTBUG-88975
Change-Id: Iff7828ddeed353620eaa9ac669a3e0c03749daa2
Reviewed-by: Paul Wicking <paul.wicking@qt.io>