Document Default Credentials
Adds a doc with a table of default creds, how to override (where applicable), and how to get if stored in secrets/configmaps.
Closes https://repo1.dso.mil/platform-one/big-bang/bigbang/-/issues/553
Preview of the doc: https://repo1.dso.mil/platform-one/big-bang/bigbang/-/blob/default-creds/docs/guides/using_bigbang/default_credentials.md
Edited by Micah Nagel