Add S3 (#37771)

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Co-authored-by: c0ffeeca7 <38767475+c0ffeeca7@users.noreply.github.com>

source/_integrations/s3.markdown

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+---
+title: S3
+description: Instructions on how to setup S3 bucket to be used as a backup location.
+ha_release: 2025.4
+ha_category:
+  - Backup
+ha_iot_class: Cloud Push
+ha_config_flow: true
+ha_domain: s3
+ha_codeowners:
+  - '@tomasbedrich'
+ha_integration_type: service
+---
+
+The **S3** {% term integrations %} allows you to use S3 bucket with Home Assistant Backups.
+
+[AWS S3](https://aws.amazon.com/s3/) is the most widely known service with S3 API.
+Any S3-compatible service (for example, [Minio](https://github.com/minio/minio)) should work with this integration as well.
+
+{% include integrations/config_flow.md %}
+
+{% configuration_basic %}
+Access Key ID:
+  description: "The access key ID for your S3 account."
+Secret Access Key:
+  description: "The secret access key for your S3 account."
+Bucket Name:
+  description: "S3 bucket name to store the backups. Bucket must already exist and be writable by the provided credentials."
+Endpoint URL:
+  description: "Endpoint URL provided to [Boto3 Session](https://boto3.amazonaws.com/v1/documentation/api/latest/reference/core/session.html). Region-specific [AWS S3 endpoints](https://docs.aws.amazon.com/general/latest/gr/s3.html) are available in their documentation. Defaults to `https://s3.eu-central-1.amazonaws.com/`."
+{% endconfiguration_basic %}
+
+## Removing the integration
+
+This integration follows standard integration removal. No extra steps are required.
+
+{% include integrations/remove_device_service.md %}