- Make custom_card_pictures.md the canonical home of the URL reference-point
table; loading_card_pictures.md cross-references it through @ref instead of
maintaining a second copy (unaddressed review comment).
- Switch the remaining @subpage custom_card_pictures to @ref in
fixing_card_pictures.md so the page keeps its single parent under
user_reference.
- Document the Image Overrides feature added in #7311/#7312 on the user page,
loading_card_pictures.md and fixing_card_pictures.md: local override storage,
the downloadedPics root lookup, exact file-name matching, and the set-folder
vs flat export naming schemes.
The actual cyclic dependency fix is converting the mutual @subpage
reference from custom_card_pictures to fixing_card_pictures into a
plain @ref, so the page hierarchy no longer loops back on itself.
