[Docs] Move documentation sources to docs/ and include CONTRIBUTING (#7584)

Rename doc/ to docs/ so GitHub picks up docs/CONTRIBUTING.md and Doxygen keeps including it, and update all references to the old layout:

- Doxygen output now goes to docs-output/, exposed by .gitignore
- docs workflow triggers on docs/** and publishes docs-output/html
- theme submodule moved to docs/doxygen/theme
- schema URLs in both XML parsers point at docs/xsd/
- README, developer docs, desktop workflow filters, .dockerignore and gcovr config updated for the new paths

The v3 card database schema is kept as docs/xsd/cards_v3.xsd.

docs/doxygen/pages/user_documentation/deck_management/creating_decks.md

@@ -0,0 +1,15 @@
+@page creating_decks Creating Decks
+
+Creating a new deck is done using either the \ref editing_decks_classic or \ref editing_decks_visual.
+
+They can be accessed either by clicking the "Create Deck" button in the TabHome screen, which will open the default deck
+editor configured in the "User Interface" -> "Deck editor/storage settings" -> "Default deck editor type" setting, or
+by selecting "Deck Editor" or "Visual Deck Editor" under the "Tabs" application menu located at the top.
+
+# Further References
+
+- See @ref editing_decks for information on how to modify the attributes and contents of a deck in the Deck Editor
+widgets.
+- See @ref exporting_decks for information on how to store and persist your deck either in-client or to external
+services.
+- See @ref importing_decks for information on how to import existing decks either in-client or from external services