[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.

.github/workflows/desktop-build.yml

@@ -15,7 +15,7 @@ on:
       - '!**.md'
       - '!.github/**'
       - '!.tx/**'
-      - '!doc/**'
+      - '!docs/**'
       - '.github/workflows/desktop-build.yml'
       - 'CMakeLists.txt'
       - 'vcpkg.json'
@@ -28,7 +28,7 @@ on:
       - '!**.md'
       - '!.github/**'
       - '!.tx/**'
-      - '!doc/**'
+      - '!docs/**'
       - '.github/workflows/desktop-build.yml'
       - 'CMakeLists.txt'
       - 'vcpkg.json'