Wrong configuration path to change checkout steps

[Physikbuddha]

https://aimeos.org/docs/2024.x/frontend/html/checkout-components/#change-checkout-steps names client/html/checkout/subparts as the configuration path to configure the checkout steps.

However, the correct configuration path should be client/html/checkout/standard/subparts.

[aimeos]

There have been a few more things that was wrong in the latest version. They are all fixed now. Thanks a lot!