Sync height for light node

[rootulp]

Context

https://docs.celestia.org/how-to-guides/light-node

Problem

This page doesn't include commands to get a trusted height for the light node so it starts syncing from genesis on Mainnet which will take a long time.

Proposal

Copy + paste the commands from the other page that shows how to get a height + hash for a recent block height and feed that to the light node to start syncing from a more recent height.

[rootulp]

Closing in favor of #1876