docs: rework book (#2566)

book/run/mainnet.md

@@ -0,0 +1,62 @@
+# Running Reth on Ethereum Mainnet or testnets
+
+Reth is an [_execution client_](https://ethereum.org/en/developers/docs/nodes-and-clients/#execution-clients). After Ethereum's transition to Proof of Stake (aka the Merge) it became required to run a [_consensus client_](https://ethereum.org/en/developers/docs/nodes-and-clients/#consensus-clients) along your execution client in order to sync into any "post-Merge" network. This is because the Ethereum execution layer now outsources consensus to a separate component, known as the consensus client. 
+
+There are multiple choices of consensus clients, but in this guide we will assume usage of Lighthouse 🦀.
+
+## Running the Reth Node
+
+First, ensure that you have Reth installed by following the [installation instructions][installation].
+
+Now, start the node as follows:
+
+```bash
+RUST_LOG=info reth node
+```
+
+> Note that this command will not open any HTTP/WS ports by default. You can change this by adding the `--http`, `--ws` flags, respectively and using the `--http.api` and `--ws.api` flags to enable various [JSON-RPC APIs](../api/api.md). For more commands, see the [`reth node` CLI reference](../cli/node.md).
+
+The EL <> CL communication happens over the [Engine API](https://github.com/ethereum/execution-apis/blob/main/src/engine/common.md), which is by default exposed at `http://localhost:8551`. The connection is authenticated over JWT using a JWT secret which is auto-generated by Reth under `~/.local/share/reth/mainnet/jwt.hex` in Linux (`/Users/<NAME>/Library/Application Support/reth/mainnet/jwt.hex` in Mac). 
+
+You can override this path using the `--authrpc.jwtsecret` option. You MUST use the same JWT secret in BOTH Reth and the chosen Consensus Layer. If you want to override the address or port, you can use the `--authrpc.addr` and `--authrpc.port` options, respectively.
+
+So one might do:
+
+```bash
+RUST_LOG=info reth node \
+    --authrpc.jwtsecret /path/to/secret \
+    --authrpc.addr 127.0.0.1 \
+    --authrpc.port 9999
+```
+
+At this point, our Reth node has started discovery, and even discovered some new peers. But it will not start syncing until you spin up the consensus layer! 
+
+## Running the Consensus Layer
+
+First, make sure you have Lighthouse installed. Sigma Prime provides excellent [installation](https://lighthouse-book.sigmaprime.io/installation.html) and [node operation](https://lighthouse-book.sigmaprime.io/run_a_node.html) instructions.
+
+Assuming you have done that, run:
+
+```bash
+RUST_LOG=info lighthouse bn \
+    --checkpoint-sync-url https://mainnet.checkpoint.sigp.io \
+    --execution-endpoint http://localhost:8551 \
+    --execution-jwt ~/.local/share/reth/jwtsecret/jwt.hex
+```
+
+Your Reth node should start receiving "fork choice updated" messages, and begin syncing the chain.
+
+## Verify the chain is growing
+
+You can easily verify that by inspecting the logs, and seeing that headers are arriving in Reth. Sit back now and wait for the stages to run!
+In the meantime, consider setting up [observability](./observability.md) to monitor your node's health or [test the JSON RPC API](../api/api.md).
+
+<!-- TODO: Add more logs to help node operators debug any weird CL to EL messages! -->
+
+[installation]: ./../installation//installation.md
+[docs]: https://github.com/paradigmxyz/reth/tree/main/docs
+[metrics]: https://github.com/paradigmxyz/reth/blob/main/docs/design/metrics.md#current-metrics
+
+## Running without a Consensus Layer
+
+We provide a method for running Reth without a Consensus Layer via the `--debug.tip <HASH>` parameter. If you provide that to your node, it will simulate sending a `engine_forkChoiceUpdated` message _once_ and will trigger syncing to the provided block hash. This is useful for testing and debugging purposes, but in order to have a node that can keep up with the tip you'll need to run a CL alongside it. At the moment we have no plans of including a Consensus Layer implementation in Reth, and we are open to including light clients other methods of syncing like importing Lighthouse as a library.