[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/documentation-build.yml

@@ -6,7 +6,7 @@ permissions:
 on:
   pull_request:
     paths:
-      - 'doc/doxygen/**'
+      - 'docs/**'
       - '.github/workflows/documentation-build.yml'
       - 'Doxyfile'
   release:
@@ -71,7 +71,7 @@ jobs:
           destination_dir: docs    # docs will be available at https://cockatrice.github.io/docs/
           external_repository: Cockatrice/cockatrice.github.io
           publish_branch: master
-          publish_dir: ./docs/html
+          publish_dir: ./docs-output/html
 
       - name: Link to Documentation Page
         if: github.event_name == 'release' || github.event_name == 'workflow_dispatch'