Spring-ai Add Generic Model documentation

Add to documentation section "Generic Model" that provides a description of the "meta api" that describes

public interface ModelClient<TReq extends ModelRequest<?>, TRes extends ModelResponse<?>> {

    /**
     * Executes a method call to the AI model.
     * @param request the request object to be sent to the AI model
     * @return the response from the AI model
     */
    TRes call(TReq request);

}

and related classes so that contributors can follow the establish patterns.

Comment From: markpollack

duplicate of #267