docs(evm): add meaningful NatSpec documentation to core EVM contracts

[awixor]

Closes #3477

What this PR does

Adds and improves NatSpec documentation across all core EVM contracts so that forge doc can auto-generate developer documentation suitable for docs.wormhole.com.

Contracts updated

• Structs.sol — documented all struct fields (VAA concept, guardian set lifecycle, ECDSA sig components, Wormhole chain ID distinction)
• Getters.sol — documented all public getters with Wormhole-specific context
• Messages.sol — added @notice/@param/@return to all public functions
• Governance.sol — documented all governance action functions
• GovernanceStructs.sol — documented all structs and parse functions, including governance action codes (1–5)
• Implementation.sol — documented publishMessage and initialize
  interfaces/IWormhole.sol — full interface documentation (primary developer-facing entrypoint)