QDoc: Separate publication choices from rendering

QDoc couples output formats to the renderer that produces them.
This makes it hard to establish which outputs a build requests and
who owns them before parsing begins. Broken choices can survive
until substantial work has begun.

Separate publication choices from rendering so later publication
work can rely on unambiguous requests and ownership. This lets
consumers reject broken declarations rather than silently choose a
different renderer. Leave generation and existing configurations
unchanged.

Task-number: QTBUG-151408
Pick-to: 6.12
Change-Id: I758096a9b0a874a7dbcd1e8c64037d47dca8f138
Reviewed-by: Topi Reinio <topi.reinio@qt.io>