docs: Path unwinding and forwarding requirements

[womensrights]

Description

This pr details the token path unwinding and token forwarding requirements document

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Summary by CodeRabbit

• New Features
  • Enhanced fungible token transfers by implementing path unwinding and token forwarding in the context of inter-blockchain communication (IBC), automating route determination and enabling token forwarding to other chains to simplify processes for end users.

[ethanfrey]

This seems related to https://github.com/confio/ics23/pull/59

I have little knowledge of buf but happy to merge if someone knows how to set this up for all 3 supported languages