Document the trade offs between using `.env` files vs `.json` files to store tokens and other authentication information

[noorbuchi]

Currently, we don't have anything discussing the difference between using either approach to store the tokens and authentication data that users get from Github and Google Service accounts. It might be worth investigating each to understand how each one is retrieved and organized in the file system, and how are they passed to SheetShuttle as CLI arguments.
Once the differences are understood, documenting them could help out users in picking. The documentation can be either in the README or in the docs directory.