feat: add parallel transactions documentation

Co-Authored-By: Kristof Gazso <[email protected]>

docs/pages/permissionless/parallel-transactions.mdx

@@ -0,0 +1,63 @@
+# Sending Multiple Transactions in Parallel
+
+When working with smart accounts, you can send multiple transactions in parallel by using custom nonce keys. This guide explains how to use this feature effectively.
+
+## Understanding Nonce Keys
+
+The EntryPoint contract implements a nonce mechanism that uses a single `uint256` nonce value in the user operation, but treats it as two values:
+- `key` (high 192 bits)
+- `seq` (low 64 bits)
+
+This allows you to:
+1. Send sequential transactions using the same key (incrementing seq)
+2. Send parallel transactions using different keys
+
+## Sending Multiple Transactions in Parallel
+
+Here's how to send multiple transactions in parallel:
+
+```typescript
+// First transaction with key 0
+const userOperation1 = await client.buildUserOperation({
+  userOperation: {
+    callData: encodeFunctionData({...}),
+  },
+})
+
+// Second transaction with key 1
+const userOperation2 = await client.buildUserOperation({
+  userOperation: {
+    callData: encodeFunctionData({...}),
+    nonce: BigInt("0x100000000000000000000000000000000000000000000000000"), // key = 1
+  },
+})
+
+// Send both transactions
+await Promise.all([
+  client.sendUserOperation({ userOperation: userOperation1 }),
+  client.sendUserOperation({ userOperation: userOperation2 }),
+])
+```
+
+## Important Considerations
+
+1. Each key maintains its own sequence counter
+2. Transactions with different keys can be processed in any order
+3. Transactions with the same key must be processed sequentially
+4. Keys must be less than 2^192
+
+## Best Practices
+
+1. Use key 0 for regular sequential transactions
+2. Use different keys for independent transaction batches
+3. Keep track of used keys to avoid conflicts
+4. Consider gas costs when sending parallel transactions
+
+## Error Handling
+
+If you encounter nonce-related errors:
+- Check if you're reusing a nonce that has already been used
+- Verify the nonce is formatted correctly
+- Ensure you're not using a nonce too far in the future (more than 10 higher than the current nonce)
+
+For more details on nonce-related errors, see the [AA25 Invalid Nonce](/permissionless/bundler/entrypoint-errors/aa25) error documentation.