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.
