message: Add link to `Types.WithdrawalTransaction` & `Types.OutputRootProof`
diff: @@ -134,6 +134,9 @@ The Optimism Portal serves as both the entry and exit point to the Optimism L2.
  the [DepositFeed](./deposits.md#deposit-contract) contract, and in addition provides the following interface for
  withdrawals:
  +- [WithdrawalTransaction type](https://github.com/ethereum-optimism/optimism/blob/develop/packages/contracts-bedrock/contracts/libraries/Types.sol#L46-L56)
  +- [OutputRootProof type](https://github.com/ethereum-optimism/optimism/blob/develop/packages/contracts-bedrock/contracts/libraries/Types.sol#L20-L29)
  +
  ```js
  interface OptimismPortal {
  