feat(docs/op-stack): Add forced withdrawals

Merge after https://github.com/ethereum-optimism/optimism-tutorial/pull/178

Closing: DEVRL-783

docs/op-stack/src/.vuepress/config.js

@@ -186,6 +186,7 @@ module.exports = {
         children: [
           '/docs/security/faq.md',
           '/docs/security/policy.md',
+          '/docs/security/forced-withdrawal.md'
         ]
       },
     ],  // end of sidebar

docs/op-stack/src/docs/security/forced-withdrawal.md

+---
+title: Forced withdrawal from an OP Stack blockchain
+lang: en-US
+---
+
+
+## What is this?
+
+Any assets you own on an OP Stack blockchain are backed by equivalent assets on the underlying L1, locked in a bridge. 
+In this article you learn how to withdraw these assets directly from L1.
+
+Note that the steps here do require access to an L2 endpoint.
+However, that L2 endpoint can be a read-only replica.
+
+
+## Setup 
+
+The code for this article is available at [our tutorials repository](https://github.com/ethereum-optimism/optimism-tutorial/tree/main/op-stack/forced-withdrawal).
+
+1. Clone the repository, move to the correct directory, and install the software you need.
+
+   ```sh
+   git clone https://github.com/ethereum-optimism/optimism-tutorial.git
+   cd op-stack/forced-withdrawal
+   npm install
+   ```
+
+1. Copy the environment setup variables.
+
+   ```sh
+   cp .env.example .env
+   ```
+
+1. Edit `.env` to set these variables:
+
+   | Variable             | Meaning |
+   | -------------------- | ------- |
+   | L1URL                | URL to L1 (Goerli if you followed the directions on this site)
+   | L2URL                | URL to the L2 from which you are withdrawing
+   | PRIV_KEY             | Private key for an account that has ETH on L2. It also needs ETH on L1 to submit transactions
+   | OPTIMISM_PORTAL_ADDR | Address of the `OptimismPortalProxy` on L1.
+
+
+## Withdrawal
+
+### ETH withdrawals
+
+The easiest way to withdraw ETH is to send it to the bridge, or the cross domain messenger, on L2.
+
+1. Enter the Hardhat console.
+
+   ```sh
+   npx hardhat console --network l1
+   ```
+
+1. Specify the amount of ETH you want to transfer.
+   This code transfers one hundred'th of an ETH.
+
+   ```js
+   transferAmt = BigInt(0.01 * 1e18)
+   ``` 
+
+
+
+1. Create a contract object for the [`OptimismPortal`](https://github.com/ethereum-optimism/optimism/blob/develop/packages/contracts-bedrock/contracts/L1/OptimismPortal.sol) contract.
+
+   ```js
+   optimismContracts = require("@eth-optimism/contracts-bedrock")
+   optimismPortalData = optimismContracts.getContractDefinition("OptimismPortal")
+   optimismPortal = new ethers.Contract(process.env.OPTIMISM_PORTAL_ADDR, optimismPortalData.abi, await ethers.getSigner())
+   ```
+
+1. Send the transaction.
+
+   ```js
+   txn = await optimismPortal.depositTransaction(
+      optimismContracts.predeploys.L2StandardBridge,
+      transferAmt,
+      1e6, false, []
+   )
+   rcpt = await txn.wait()
+   ```
+
+
+1. To [prove](https://sdk.optimism.io/classes/crosschainmessenger#proveMessage-2) and [finalize](https://sdk.optimism.io/classes/crosschainmessenger#finalizeMessage-2) the message we need the hash. 
+   Optimism's [core-utils package](https://www.npmjs.com/package/@eth-optimism/core-utils) has the necessary function.
+
+   ```js
+   optimismCoreUtils = require("@eth-optimism/core-utils")
+   withdrawalData = new optimismCoreUtils.DepositTx({
+      from: (await ethers.getSigner()).address,
+      to: optimismContracts.predeploys.L2StandardBridge,
+      mint: 0,
+      value: ethers.BigNumber.from(transferAmt),
+      gas: 1e6,
+      isSystemTransaction: false,
+      data: "",
+      domain: optimismCoreUtils.SourceHashDomain.UserDeposit,
+      l1BlockHash: rcpt.blockHash,
+      logIndex: rcpt.logs[0].logIndex,
+   })
+   withdrawalHash = withdrawalData.hash()
+   ```
+
+
+1. Create [a cross domain messenger](https://sdk.optimism.io/classes/crosschainmessenger).
+   This step, and subsequent ETH withdrawal steps, are explained in [this tutorial](https://github.com/ethereum-optimism/optimism-tutorial/tree/main/cross-dom-bridge-eth).
+
+   ```js
+   optimismSDK = require("@eth-optimism/sdk")
+   l2Provider = new ethers.providers.JsonRpcProvider(process.env.L2URL)
+   await l2Provider._networkPromise
+   crossChainMessenger = new optimismSDK.CrossChainMessenger({
+      l1ChainId: ethers.provider.network.chainId,
+      l2ChainId: l2Provider.network.chainId,
+      l1SignerOrProvider: await ethers.getSigner(),
+      l2SignerOrProvider: l2Provider,
+      bedrock: true
+   })   
+   ```
+
+1. Wait for the message status for the withdrawal to become `READY_TO_PROVE`.
+   By default the state root is written every four minutes, so you're likely to need to need to wait.
+
+   ```js
+   await crossChainMessenger.waitForMessageStatus(withdrawalHash, 
+       optimismSDK.MessageStatus.READY_TO_PROVE)
+   ```
+      
+1. Submit the withdrawal proof.
+
+   ```js
+   await crossChainMessenger.proveMessage(withdrawalHash)
+   ```
+
+1. Wait for the message status for the withdrawal to become `READY_FOR_RELAY`.
+   This waits the challenge period (7 days in production, but a lot less on test networks).
+
+   ```js
+   await crossChainMessenger.waitForMessageStatus(withdrawalHash, 
+      optimismSDK.MessageStatus.READY_FOR_RELAY)
+   ```   
+
+
+1. Finalize the withdrawal.
+   See that your balance changes by the withdrawal amount.
+
+   ```js
+   myAddr = (await ethers.getSigner()).address
+   balance0 = await ethers.provider.getBalance(myAddr)
+   finalTxn = await crossChainMessenger.finalizeMessage(withdrawalHash)
+   finalRcpt = await finalTxn.wait()
+   balance1 = await ethers.provider.getBalance(myAddr)
+   withdrawnAmt = BigInt(balance1)-BigInt(balance0)
+   ```
+
+::: tip transferAmt > withdrawnAmt
+
+Your L1 balance doesn't increase by the entire `transferAmt` because of the cost of `crossChainMessenger.finalizeMessage`, which submits a transaction.
+
+:::
\ No newline at end of file