Refactor docs

docs/s3.md

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+# S3 Compabible Object Storage backends
+
+Specifying a different `key_prefix`, you can assign different virtual folders of the same bucket to different users. This is similar to a chroot directory for local filesystem. Each SFTP/SCP user can only access the assigned virtual folder and its contents. The virtual folder identified by `key_prefix` does not need to be pre-created.
+
+SFTPGo uses multipart uploads and parallel downloads for storing and retrieving files from S3.
+
+The configured bucket must exist.
+
+To connect SFTPGo to AWS, you need to specify credentials, and a `region` is required too. Here is the list of available [AWS regions](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-regions-availability-zones.html#concepts-available-regions). For example, if your bucket is at `Frankfurt`, you have to set the region to `eu-central-1`. You can specify an AWS [storage class](https://docs.aws.amazon.com/AmazonS3/latest/dev/storage-class-intro.html) too. Leave it blank to use the default AWS storage class. An endpoint is required if you are connecting to a Compatible AWS Storage such as [MinIO](https://min.io/).
+
+AWS SDK has different options for credentials. [More Detail](https://docs.aws.amazon.com/sdk-for-go/v1/developer-guide/configuring-sdk.html). We support:
+1. Providing [Access Keys](https://docs.aws.amazon.com/general/latest/gr/aws-sec-cred-types.html#access-keys-and-secret-access-keys).
+2. Use IAM roles for Amazon EC2
+3. Use IAM roles for tasks if your application uses an ECS task definition
+
+So, you need to provide access keys to activate option 1, or leave them blank to use the other ways to specify credentials.
+
+Some SFTP commands don't work over S3:
+
+- `symlink` and `chtimes` will fail
+- `chown` and `chmod` are silently ignored
+- upload resume is not supported
+- upload mode `atomic` is ignored since S3 uploads are already atomic
+
+Other notes:
+
+- `rename` is a two step operation: server-side copy and then deletion. So, it is not atomic as for local filesystem.
+- We don't support renaming non empty directories since we should rename all the contents too and this could take a long time: think about directories with thousands of files; for each file we should do an AWS API call.
+- For server side encryption, you have to configure the mapped bucket to automatically encrypt objects.
+- A local home directory is still required to store temporary files.