Expand documentation

[r4gus]

Since we now stick to the stable releases of Zig, I think it's time to expand the documentation/ README. Questions the docs should answer:

1. How to get started/ setup a minimal project
2. How to flash the pico
   • uf2 via DND interface
   • elf via debugger
3. Maybe an explanation of the projects structure (where to find what)
4. How to contribute guidelines (this belong probably more in the microzig repo)