Improvements to polkadot-sdk-frame umbrella crate

[kianenigma]

• In the docs, explicitely mention the suggested use of prelude modules as the suggested way. Atm, this is not easily visibile in the first example that you would encounter, if you start digging into the crate docs.
• Provide proper erros for #[cfg(not(feature = "experimental"))] that, instead of not working, provide a compiler error that says the frame crate is experimental.

[kianenigma]

• Make preludes use-able: As in: I should be able to write a reasonable pallet + runtime + tests + benchmarks without needing to use anything other than prelude::* + runtime::prelude::* + testing::prelude::* + benchmarking::prelude::*.
• Either remove the experimental flag, or document the issues that need to be removed before we remove it.

[shawntabrizi]

You are not exporting TokenError, and perhaps other DispatchError variants.