Missing instructions regarding mev-boost and external lighthouse consensus client

[dsk7]

Hi folks - thanks for your great work on the docs! 👍

Issue

It seems to me that in order to use mev-boost and lighthouse as an external consensus client, one needs to expose the mev-boost API via rocketpool service config and specify --builder $URL_OF_MEV_BOOST in the command line options of the external lighthouse consensus client. It took me some time to find that out and I was unable to find a solution via google or the docs.

Proposed solution

It would be nice if the documentation would mention this somewhere.

Background

When setting up a rocketpool node according to the docs and having lighthouse as an external consensus client I stumbled upon an issue I was unable to find help with:
Regularly, the validator would log:
WARN Unable to publish validator registrations to the builder network, error: Some endpoints failed, num_failed: 1 $URL_OF_CONSENSUS => RequestFailed(ServerMessage(ErrorMessage { code: 500, message: "UNHANDLED_ERROR: BuilderMissing", stacktraces: [] })), service: preparation
While investigating this, I found a note the lighthouse docs at https://lighthouse-book.sigmaprime.io/builders.html that the consensus client needs a command line option providing the URL of the mev-boost relay. Doing this made the log message change to INFO Published validator registrations to the builder network, count: 3, service: preparation. So everything looks fine now.

###Notes
$URL_OF_MEV_BOOST and $URL_OF_CONSENSUS are placeholders I used for the real URLs.