Add instructions for authenticating using the AWS CLI and deleting files

[tylere]

https://docs.source.coop/data-proxy#using-the-aws-cli currently only describes unauthenticated access, and listing of directories.

Add instructions for authenticating and modifying the contents (upload, delete, etc.)

For example:

1. View Credentials
2. Switch to the Environment Variables tab, and copy the export statements.
3. Open a terminal
4. Paste in the export statements
5. Run a command that modifies the files in the bucket:

aws s3 rm s3://us-west-2.opendata.source.coop/tyler/test-files/ --recursive