Add support for platform docs

Ensures we can pick up changes introduced in https://github.com/godotengine/godot/pull/76251

.github/workflows/sync_class_ref.yml

@@ -48,7 +48,7 @@ jobs:
 
       - name: Build new documentation
         run: |
-          ./.engine-src/doc/tools/make_rst.py --color -o ./classes -l en ./.engine-src/doc/classes ./.engine-src/modules
+          ./.engine-src/doc/tools/make_rst.py --color -o ./classes -l en ./.engine-src/doc/classes ./.engine-src/modules ./.engine-src/platforms
 
       - name: Submit a pull-request
         uses: peter-evans/create-pull-request@v5