✨(yhub) allow optional usage of s3 persistent plugin

Yhub has a s3 persistent plugin, since now we didn't use it but we wanto
to give the possibility to use it optionnaly. To enable it set the
YHUB_S3_PERSISTENCE environment value to true and configure it.

env.d/development/yhub

@@ -32,3 +32,15 @@ SOFT_MIGRATION=true
 LEGACY_S3_ENDPOINT_URL=http://minio:9000
 LEGACY_S3_ACCESS_KEY_ID=impress
 LEGACY_S3_SECRET_ACCESS_KEY=password
+
+# Document storage: where the blobs of a compaction are written. Off, they stay
+# in yhub's postgres, which is what this stack runs. Turning it on stores them
+# in object storage instead — here the same minio, in a bucket of its own that
+# the server creates on startup when it is missing. Read the "Document storage"
+# section of src/yhub-server/README.md first: a document persisted this way
+# cannot be read back with the plugin turned off again.
+YHUB_S3_PERSISTENCE=false
+YHUB_S3_ENDPOINT_URL=http://minio:9000
+YHUB_S3_ACCESS_KEY_ID=impress
+YHUB_S3_SECRET_ACCESS_KEY=password
+YHUB_S3_BUCKET_NAME=yhub-storage