QDoc: Decouple index serialization location inputs

QDoc's index serializer, which also writes WebXML pages, asks a
legacy output generator where each document and example file
lives. It checks the output format's name to decide whether to
list example files inline. It reads inclusion and source-location
settings from global configuration. When the caller names no
generator, index writing falls back to the HTML one on its own.
Every index therefore depends on a legacy renderer, even when
another pipeline produces the documentation. Link locations can't
evolve without changing the serializer itself.

Let the caller supply complete link targets along with the
serialization choices. These cover which declarations to include,
whether to record source locations, whether example files appear
inline, and which project the index belongs to. The HTML and
WebXML rules become adapters owned by their callers. The step that
writes the index decides which renderer supplies its links.

Paths, anchors, and XML remain unchanged. Location planning can
evolve independently, while the legacy adapters stay in place
during the migration to the template-based pipeline.

Pick-to: 6.12
Fixes: QTBUG-151407
Change-Id: Ife28d6f7bc8b69db5ee66d655cad69512ce2dcf6
Reviewed-by: Topi Reinio <topi.reinio@qt.io>