qt6_tool: Add documentation build


Factor out some helper functions and add an option
for a doc only build in a folder named "doc" under the build
directory.

Change-Id: I34a22be2634acfa787309078dbad16d46738a564
Reviewed-by: Cristian Maureira-Fredes <cristian.maureira-fredes@qt.io>
commit5249ca1728366f1026ff367c7013081e0a5b8ddb
parentc213a128b3d1c554e980886deaf9dca61fd6e919
authorFriedemann Kleint <Friedemann.Kleint@qt.io>
committerFriedemann Kleint <Friedemann.Kleint@qt.io>
commit timeThu, 23 Jul 2026 14:53:12 +0000