docs: write up content-type docs

Area of Improvement

https://github.com/trpc/trpc/pull/5613 implements content-type handlers for FormData and Octet data, but only in specific adapters, and for v11 we need docs for all this

• We need docs for the new content-type APIs
• We need errors/doc comments to point at docs
• We need a compatibility table for various content types
• We need to update the RPC spec: https://trpc.io/docs/rpc

Link to related docs

No response

Additional information

No response

‍‍ Contributing

• ‍♂️ Yes, I'd be down to file a PR implementing the suggested changes!