Add information on how to use stellar-etl to obtain historical data to hubble docs

[janewang]

What problem does your feature solve?

Looking for a word doc or PR on ingesting historical data
[ ] How to use stellar-etl to obtain historical data from hubble
[ ] Discuss ledger exporter as a potential future option

[ElliotFriend]

#669 Seems to have accomplished this. @janewang If the content here doesn't quite fit what you have in mind, feel free to reopen the issue.