qtattributionsscanner: Suppress QDoc warnings for TOC inclusion

QDoc issues warnings for generated pages that are not included in the
<toc> structure of the Qt Help Project (.qhp) file. Suppress these
warnings for attribution pages: In Qt framework documentation, these
pages are listed via their respective group name in a central location
for all Qt modules. Each module is not expected to include them in
their own table of contents.

Task-number: QTBUG-149176
Change-Id: Iafab01f0cdf681bcf6bf547d5c98fe1b03086a05
Reviewed-by: Paul Wicking <paul.wicking@qt.io>