feat: document mainnet.sh script in celestia-app

[jcstein]

          Notes from @rootulp on how to view logs:

View log file:

cat mainnet.log

View actively running process which is piping into a log file:

tail -f mainnet.log

To stop the process:

ps aux | grep celestia-app
kill <PID>

# if that doesnt' work
kill -9 <PID>

Originally posted by @jcstein in celestiaorg/celestia-app#3166 (comment)

[Bidon15]

this is not ok to put into docs.
Ideally, this should be a community support doc(i.e. medium blog post or smt similar) that is hosted by the 3rd party (any validator team or rpc team, etc.)