fix: correct port references in README and add commented ports section

- README: use 3456 instead of 3000
- docker-compose.yml: add commented ports for optional direct access

docker-compose.yml

@@ -31,6 +31,8 @@ services:
 
   vikunja:
     image: vikunja/vikunja:0.24
+    # ports:
+    #   - 3456:3456
     environment:
       VIKUNJA_DATABASE_TYPE: postgres
       VIKUNJA_DATABASE_HOST: 127.0.0.1