Add user render passes overview documentation page

Add qtquick3d-userpasses.qdoc as an overview page for the user-defined
render passes API introduced in 6.11. Covers core concepts, examples,
and links to API reference documentation.

Updates qtquick3d-toc.qdoc and qtquick3d-custom.qdoc to link to the new page.

Pick-to: 6.11
Change-Id: I9d2365a80dc56c7b6c710115f26ac05bbfed1708
Reviewed-by: Christian Strømme <christian.stromme@qt.io>