Document metadata format

[cmichi]

We should document the metadata format for a stable 3.0 release. Without documentation the hurdle to build third party tools (like different UI's or CLI tools) is too high.

Ideally the ink-docs would contain a section on the metadata as well, maybe linking to more extensive documentation in the ink_metadata crate?

Once we have this documentation we should also communicate it to stakeholders (Patract, etc.).

[yjhmelody]

@Robbepop Hi, I found polkadot apps started to use new metadata format(My metadata cannot be uploaded to apps).

PortableRegistry: 3: Error extracting {"path":[],"params":[],"def":{"composite":{"fields":[{"name":"inner","type":2,"typeName":"FixedArray32<u8>","docs":[]}]}},"docs":[]}: Cannot read properties of undefined (reading 'toString')

What's the changes ?
I thought it was just changing the index from 1 to 0, but it doesn’t seem to be.

[cmichi]

@ascjones I just merged #923, polkadot-js/api has been updated in polkadot-js/api#4289. So this issue is no longer blocked.