Provide some raw material for high-level documentation

[lsgunnlsgunn]

General description of the RUST CDK Description and use case example for each macro. (The detailed API is going to be extracted from the code using docs.rs as I understand it, but I think we still need some higher-level information to make sense of the reference information).

[chmllr]

I'd like to stress here, that we should also spend a paragraph or two on the concurrency model (or the total absence of it within a single function call), so that it explains, why using static mut in the officially provided examples is fine.