[Content] Adds docs about construct_runtime!

[bkchr]

Content request

We need better docs for the macro. This includes the syntax and especially what kind of types the macro generates.

Are you willing to help with this request?

Yes!

[sacha-l]

Notes to self:

construct_runtime generates:

• Call enum, used for libraries like subxt to submit extrinsics to the runtime.
• Event, origin enums generated to
• Pallet indices are generated
• Pallet name: useful to ensure storage ValueQuery is unique
• get_call_metadata
• GenesisConfig is generated and used in the chainspec.
• InherentDataExt: ensure that inherents are applied before the extrinsics
• ValidateUnsigned
• ...

(run subxt codegen | rustfmt --edition=2018 --emit=stdout and see. Or cargo expand to break it down).