Add a copy + pasteable command for `celestia light start` on mainnet beta

[rootulp]

Context

https://docs.celestia.org/nodes/light-node#start-the-light-node

Problem

There isn't an example command that I can copy + paste to start a light node on mainnet

Proposal

1. Add an additional command that I can copy + paste to start a light node on mainnet.
2. [optional] use tabs like this:

3. [optional] modify the heading level so that start the light node is at the same level as Initialize the node