Update s3-store docs to storage after %garden deprecation.

[sigilante]

Tlon has advertised changes to s3-store. This is currently documented at https://operators.urbit.org/manual/os/s3

@mcevoypeter who should be on point for this?

[mcevoypeter]

@jamesacklin is the person who'll facilitate communication around the s3-store changes. I'd ask him.