docs: move C++ API docs to subfolder

docs/cpp-api/assets/doxygen-awesome-css/doxygen-custom/toggle-alternative-theme.js

@@ -0,0 +1,12 @@
+
+let original_theme_active = true;
+
+function toggle_alternative_theme() {
+    if(original_theme_active) {
+        document.documentElement.classList.add("alternative")
+        original_theme_active = false;
+    } else {
+        document.documentElement.classList.remove("alternative")
+        original_theme_active = true;
+    }
+}