feat(book): Consensus (#212)

### Description

Documents the `op-alloy-consensus` crate in the mdbook.

book/src/SUMMARY.md

@@ -6,6 +6,8 @@
   - [Genesis](./building/genesis/README.md)
     - [Rollup Config](./building/genesis/rollup-config.md)
     - [System Config](./building/genesis/system-config.md)
+  - [Consensus](./building/consensus/README.md)
+    - [Transactions](./building/consensus/transactions.md)
 - [Examples](./examples/README.md)
 - [Contributing](./CONTRIBUTING.md)
 - [Licensing](./LICENSE.md)

book/src/building/consensus/transactions.md

@@ -0,0 +1,77 @@
+# Transactions
+
+The [`op-alloy-consensus`][consensus] crate contains types for Optimism EL
+consensus and communication. Most notably, Optimism extends the Ethereum
+[EIP-2718][2718] transaction envelope to include a deposit variant.
+
+This doc breaks down transaction and other types defined in the
+[`op-alloy-consensus`][consensus] crate.
+
+## [`OpTxEnvelope`][envelope]
+
+The [`OpTxEnvelope`][envelope] type is based on [Alloy][alloy]'s
+[`TxEnvelope`][tx-envelope] type.
+
+Optimism modifies the `TxEnvelope` to the following.
+- Legacy
+- EIP-2930
+- EIP-1559
+- EIP-7702
+- Deposit
+
+Deposit is a custom transaction type that is either an L1 attributes
+deposit transaction or a user-submitted deposit transaction. Read more
+about deposit transactions in [the specs][specs].
+
+## Transaction Types ([`OpTxType`][ty])
+
+The [`OpTxType`][ty] enumerates the transaction types using their byte identifier,
+represents as a `u8` in rust.
+
+## [`OpBlock`][block]
+
+[`op-alloy-consensus`][consensus] exports an Optimism block type.
+
+## Receipt Types
+
+Just like [`op-alloy-consensus`][consensus] defines transaction types,
+it also defines associated receipt types.
+
+[`OpReceiptEnvelope`][ore] defines an [Eip-2718][2718] receipt envelope type
+modified for the OP Stack. It contains the following variants - mapping
+directly to the `OpTxEnvelope` variants defined above.
+
+- Legacy
+- EIP-2930
+- EIP-1559
+- EIP-7702
+- Deposit
+
+There is also an [`OpDepositReceipt`][odr] type, extending the alloy receipt
+type with a deposit nonce and deposit receipt version.
+
+## Hardforks
+
+Aside from transactions and receipts, [`op-alloy-consensus`][consensus] exports
+one other core primitive called [`Hardforks`][hardforks].
+
+Hardforks provides hardfork transaction constructors - that is, it provides
+methods that return upgrade transactions for each hardfork. Some of these
+are the following.
+
+- [`Hardforks::ecotone_txs()`][ecotone]
+- [`Hardforks::fjord_txs()`][fjord]
+
+
+[fjord]: https://docs.rs/op-alloy-consensus/latest/op_alloy_consensus/hardforks/struct.Hardforks.html#method.fjord_txs
+[ecotone]: https://docs.rs/op-alloy-consensus/latest/op_alloy_consensus/hardforks/struct.Hardforks.html#method.ecotone_txs
+[hardforks]: https://docs.rs/op-alloy-consensus/latest/op_alloy_consensus/hardforks/struct.Hardforks.html
+[odr]: https://docs.rs/op-alloy-consensus/latest/op_alloy_consensus/struct.OpDepositReceipt.html
+[ore]: https://docs.rs/op-alloy-consensus/latest/op_alloy_consensus/enum.OpReceiptEnvelope.html
+[block]: https://docs.rs/op-alloy-consensus/latest/op_alloy_consensus/type.OpBlock.html
+[ty]: https://docs.rs/op-alloy-consensus/latest/op_alloy_consensus/enum.OpTxType.html
+[specs]: https://specs.optimism.io/protocol/deposits.html
+[tx-envelope]: https://docs.rs/alloy-consensus/latest/alloy_consensus/transaction/enum.TxEnvelope.html
+[envelope]: https://docs.rs/op-alloy-consensus/latest/op_alloy_consensus/enum.OpTxEnvelope.html
+[2718]: https://eips.ethereum.org/EIPS/eip-2718
+[consensus]: https://crates.io/crates/op-alloy-consensus