added `custom storage` to sidebar
* [[S3 storage|Configuration-Storage#s3-storage]]
* [[Etcd storage|Configuration-Storage#etcd-storage]]
* [[In memory storage|Configuration-Storage#in-memory-storage]]
+ * [[Custom storage|Configuration-Storage#custom-storage]]
* [[Storage Aliases|Configuration-Storage#storage-aliases]]
* [[Credentials|Configuration-Credentials]]
* [[File credentials|Configuration-Credentials#credentials-type-file]]
