Zephyr as main platform

[tolauwae]

We want to move to zephyr as the main platform instead of Arduino.
This means, CI testing, clear install instructions, support for all current primitives, support for all currently supported hardware, and updated documentation.

Towards the future, it might be good for maintainability to phase out all other platforms. On the other hand, multiple platforms was a core reason to choose for WebAssembly.