Rust doc user journey: Chain-spec

[kianenigma]

The concept of what a chain-spec is, which fields it has, and how to generate it should be one of the main user journeys to be covered in the rust-docs.

Some scattered information exists here, but it needs to be improved, and linked better: https://paritytech.github.io/substrate/master/sc_chain_spec/index.html#substrate-chain-specification-format

[sacha-l]

@kianenigma - was this created anywhere?