What are some unconventional methods for setting up a Zwift system on a budget, considering the cost of a smart trainer can be a significant barrier to entry, and are there any alternatives to traditional smart trainers that can still provide a reliable and immersive indoor cycling experience, such as using a power meter and a cadence sensor in conjunction with a wheel-on trainer or a DIY solution involving a speed sensor and a resistance unit, and how do these alternatives compare to traditional smart trainers in terms of accuracy, reliability, and overall user experience.