[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/README.md

@@ -0,0 +1,28 @@
+## Building Doxygen Documentation
+
+We use [Doxygen](https://www.doxygen.nl) for our code documentation.  
+After installing the tool, you can build a HTML webpage into the `/docs-output` folder according to our [configuration](https://github.com/Cockatrice/Cockatrice/blob/master/Doxyfile):
+```bash
+doxygen Doxyfile
+```
+Open the `index.html` file in the `/docs-output/html/` folder to browse the result locally.  
+There is also an online version we host at the Cockatrice webpage: [cockatrice.github.io/docs](https://cockatrice.github.io/docs)
+
+Doxygen's own documenation can be found at [www.doxygen.nl/manual](https://www.doxygen.nl/manual).
+
+<br>
+
+## Doxygen Documentation Theme
+
+Required changes to the `Doxyfile` config from the [theme docs](https://jothepro.github.io/doxygen-awesome-css/index.html#autotoc_md16) to make [doxygen-awesome-css](https://github.com/jothepro/doxygen-awesome-css) work:
+```
+HTML_EXTRA_STYLESHEET  = doxygen-awesome.css      # Main CSS file of the theme
+GENERATE_TREEVIEW      = YES                      # Optional, also works without
+HTML_COLORSTYLE        = LIGHT                    # Required with doxygen-awesome-css theme, Auto Dark Mode will still work
+DISABLE_INDEX          = NO                       # YES is bugged in the theme, see jothepro/doxygen-awesome-css/issues/201
+FULL_SIDEBAR           = NO                       # Required for doxygen-awesome-css theme
+```
+
+<br>
+
+Cockatrice dedicated color adjustments are configured in the [cockatrice_docs_style.css](https://github.com/Cockatrice/Cockatrice/blob/master/docs/doxygen/css/cockatrice_docs_style.css) file that uses & overrides the theme definitions.