QDoc: Make publication destinations predictable

Publication paths depend on inherited settings and on whether QDoc
processes modules together or separately. This makes it hard to
predict where outputs belong and to identify conflicting or unsafe
destinations before generation starts.

Give each format a stable destination across changes to requested
formats and execution mode. This gives later publication work a
predictable layout and configuration errors that point to their
cause. Keep generation and compatibility behavior unchanged;
connecting these choices to rendering remains separate work.

Fixes: QTBUG-151408
Pick-to: 6.12
Change-Id: Ic8eb69e23b71bd680b3c688bcc8442e57421dadc
Reviewed-by: Topi Reinio <topi.reinio@qt.io>