Doxygen: Improve file structure and includes (#6344)

doc/doxygen/extra-pages/developer_documentation/querying_the_card_database.md

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+@page querying_the_card_database Querying the Card Database
+
+# The CardDatabaseQuerier Class
+
+The CardDatabaseQuerier is the only class used for querying the database. The CardDatabase is an in-memory map and thus
+provides no structured query language. CardDatabaseQuerier offers methods to retrieve cards by name, by providerID or in
+bulk, as CardSet%s.
+
+## Obtaining a handle to the CardDatabaseQuerier for usage
+
+To obtain the CardDatabaseQuerier related to the global CardDatabase singleton, use CardDatabaseManager::query().
+
+## Querying for known cards
+
+There are, essentially, two ways to ensure card equality, with the second being optional but necessitating the first.
+These two ways are CardInfo name equality and PrintingInfo provider ID equality.
+
+Because of this, most queries require, at the very least, a card name to match against and optionally a providerID to
+narrow the results.
+
+### Generic Card Infos
+
+To check if a card with the exact provided name exists as a CardInfo in the CardDatabase use,
+CardDatabaseQuerier::getCardInfo() or CardDatabaseQuerier::getCardInfos() for multiple cards.
+
+### Guessing Cards
+
+If the exact name might not be present in the CardDatabase, you can use CardDatabaseQuerier::getCardBySimpleName(),
+which automatically simplifies the card name and matches it against simplified card names in the CardDatabase.
+
+Alternatively, you can use CardDatabaseQuerier::lookupCardByName(), which first attempts an exact match search and then
+uses CardDatabaseQuerier::getCardBySimpleName() as a fallback.
+
+### ExactCard%s
+
+To obtain an ExactCard from the CardDatabaseQuerier, you must use a CardRef as a parameter to
+CardDatabaseQuerier::getCard(), CardDatabaseQuerier::getCards(), or CardDatabaseQuerier::guessCard().
+
+CardRef is a simple struct consisting of a card name and a card provider ID as QString%s.