chore(book): Load Rollup Config Example (#224)

### Description

Loading a rollup config example.

book/src/SUMMARY.md

@@ -8,6 +8,7 @@
     - [System Config](./building/genesis/system-config.md)
   - [Consensus](./building/consensus.md)
 - [Examples](./examples/README.md)
+  - [Load a Rollup Config](./examples/load-a-rollup-config.md)
 - [Contributing](./CONTRIBUTING.md)
 - [Licensing](./LICENSE.md)
 - [Glossary](./glossary.md)

book/src/examples/README.md

@@ -2,4 +2,4 @@
 
 Examples for working with `op-alloy-*` crates.
 
-TODO: document `op-alloy` crate use and using it with re-exports.
+- [Load a Rollup Config for a Chain ID](./load-a-rollup-config.md)

book/src/examples/load-a-rollup-config.md

@@ -0,0 +1,34 @@
+# Loading a Rollup Config from a Chain ID
+
+In this section, the code examples demonstrate loading the
+rollup config for the given L2 Chain ID.
+
+Let's load the Rollup Config for OP Mainnet which hash chain id 10.
+
+```rust
+use op_alloy_genesis::{OP_MAINNET_CONFIG, rollup_config_from_chain_id};
+
+// The chain id for OP Mainnet
+let op_mainnet_id = 10;
+
+// Load a rollup config from the chain id.
+let op_mainnet_config = rollup_config_from_chain_id(op_mainnet_id).expect("infallible");
+
+// The chain id should match the hardcoded chain id.
+assert_eq!(OP_MAINNET_CONFIG, op_mainnet_config);
+```
+
+> ⚠️ Available Configs
+>
+> The `rollup_config_from_chain_id` method in `op-alloy-genesis` uses hardcoded
+> rollup configs. But, there are only a few of these hardcoded rollup configs in
+> `op-alloy-genesis`. This method and these configs are provided for `no_std`
+> environments where dynamic filesystem loading at runtime is not supported
+> in `no_std` environments.
+>
+> In a `std` environment, the [superchain][superchain] crate may be used which
+> dynamically provides all rollup configs from the [superchain-registry][registry]
+> for their respective chain ids.
+
+[superchain]: https://crates.io/crates/superchain
+[registry]: https://github.com/ethereum-optimism/superchain-registry