Ref docs for `runtime_vs_smart_contract`

[juangirini]

Create the reference docs as part of the Developer Hub -> https://github.com/paritytech/polkadot-sdk/blob/9c2b3eaa8c05f4f25809e87f1890679261cd1e03/developer-hub/src/reference_docs/mod.rs#L37

The focus should be: Learn about the differences between smart contracts and a FRAME-based runtime. They are both "code stored onchain", but how do they differ?