[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/html/footer.html

@@ -0,0 +1,27 @@
+<!-- HTML footer for doxygen 1.14.0-->
+<!-- start footer part -->
+<script src="$relpath^graph_toggle.js"></script>
+<!--BEGIN GENERATE_TREEVIEW-->
+<div class="navpath" id="nav-path"><!-- id is needed for treeview function! -->
+    <ul>
+        $navpath
+        <li class="footer">$generatedby <a href="https://www.doxygen.org/index.html"><img alt="doxygen"
+                                                                                          class="footer"
+                                                                                          height="31" src="$relpath^doxygen.svg"
+                                                                                          width="104"/></a>
+            $doxygenversion
+        </li>
+    </ul>
+</div>
+<!--END GENERATE_TREEVIEW-->
+<!--BEGIN !GENERATE_TREEVIEW-->
+<hr class="footer"/>
+<address class="footer"><small>
+    $generatedby&#160;<a href="https://www.doxygen.org/index.html"><img alt="doxygen" class="footer"
+                                                                        height="31" src="$relpath^doxygen.svg" width="104"/></a>
+    $doxygenversion
+</small></address>
+</div><!-- doc-content -->
+<!--END !GENERATE_TREEVIEW-->
+</body>
+</html>