External site information is needlessly duplicated for each doc module

The global Qt doc configuration includes the following line:

sources += externalsites

which adds 256 \externalpage entries into each documentation project that uses the global Qt doc config. These entries are written to every index file, leading to unneccessary duplication, increased .index file sizes, and affects the total doc generation time.

Consider the QtDoc project, for example: It has 49 dependencies, which means that the same external site information is read 50 times (100 times total over -prepare and -generate phases), and qdoc ends up having (49 * 256 = 12544) redundant entries in its node tree.