docs: How to gather s3 contents in drenv

[nirs]

Adding here until we automate this in kubectl-gather:

for obj in $(mc find dr1/bucket); do mkdir -p $(dirname s3/$obj); mc cat "$obj" > s3/$obj; done
for obj in $(mc find dr2/bucket); do mkdir -p $(dirname s3/$obj); mc cat "$obj" > s3/$obj; done

Note: I tried simpler mc command but they failed:

• mc cp -r dr1/bucket
• mc find dr1/bucket --exec "mc cp {} s3/{}"