Doc: Collect external links into a single file

Move external links to a dedicated .qdoc file for maintainability.

Add an \externalpage that's marked internal to work around the
issue of each occurrence of 'WebEngine' being autolinked to
the WebEngine QML type, which is typically not desired.

Change-Id: Icfdbefac7372866f5258378aa59daba1a97cc776
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>