Add guide to MEV logs (#3611)

## Proposed Changes

Add some docs on checking the builder configuration, which is a frequently asked question on Discord.

## Additional Info

My text editor also insisted on stripping some trailing newlines, but can put 'em back if we want

book/src/builders.md

@@ -171,6 +171,57 @@ most valuable payload offered by builders is only 0.1 ETH, the local execution e
 this threshold just looks at the value of the external payload. No comparison to the local payload is made, although
 this feature will likely be added in the future.
 
+## Checking your builder config
+
+You can check that your builder is configured correctly by looking for these log messages.
+
+On start-up, the beacon node will log if a builder is configured:
+
+```
+INFO Connected to external block builder
+```
+
+At regular intervals the validator client will log that it successfully registered its validators
+with the builder network:
+
+```
+INFO Published validator registrations to the builder network
+```
+
+When you succesfully propose a block using a builder, you will see this log on the beacon node:
+
+```
+INFO Successfully published a block to the builder network
+```
+
+If you don't see that message around the time of your proposals, check your beacon node logs
+for `INFO` and `WARN` messages indicating why the builder was not used.
+
+Examples of messages indicating fallback to a locally produced block are:
+
+```
+INFO No payload provided by connected builder.
+```
+
+```
+WARN Unable to retrieve a payload from a connected builder
+```
+
+```
+INFO The value offered by the connected builder does not meet the configured profit threshold.
+```
+
+```
+INFO Due to poor chain health the local execution engine will be used for payload construction.
+```
+
+In case of fallback you should see a log indicating that the locally produced payload was
+used in place of one from the builder:
+
+```
+INFO Reconstructing a full block using a local payload
+```
+
 [mev-rs]: https://github.com/ralexstokes/mev-rs
 [mev-boost]: https://github.com/flashbots/mev-boost
 [gas-limit-api]: https://ethereum.github.io/keymanager-APIs/#/Gas%20Limit