Guide: migrating from OSS to Cloud using BACKUP/RESTORE commands

[Blargian]

Summary

Adds a guide on how to migrate from OSS to Cloud using BACKUP/RESTORE commands.

Also splits out the guide on how to set up an S3 bucket and IAM role into it's own page instead of a snippet and updates the images. The previous one is quite outdated.

Backup and restore docs are improved here: #4049

Checklist

• Delete items not relevant to your PR
• URL changes should add a redirect to the old URL via https://github.com/ClickHouse/clickhouse-docs/blob/main/docusaurus.config.js
• If adding a new integration page, also add an entry to the integrations list here: https://github.com/ClickHouse/clickhouse-docs/blob/main/docs/integrations/index.mdx

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Project	Deployment	Preview	Comments	Updated (UTC)
clickhouse-docs	Ready	Preview		Dec 1, 2025 2:20pm
clickhouse-docs-ru	Error			Dec 1, 2025 2:20pm
clickhouse-docs-zh	Error			Dec 1, 2025 2:20pm

[dhtclk]

Hey Shaun - I've reviewed this and think we need to do some rewording and restructuring to align with docs standards for new or updated guides. A few things I'm thinking:

1. Writing style updates (Google style guide - conversational tone, contractions)
2. Better organization/flow in a few sections
3. Using vertical stepper for the main steps

Happy to discuss or pair on this.