Document turbopuffer namespace branching

[skreborn]

Resolves INT-506.

[linear-code]

INT-506

[greptile-apps]

This PR should not merge until the documented turbopuffer configuration is aligned with a CLI release that accepts the engine type.

Findings

1.  Unsupported turbopuffer config type ▶

Fix with agent prompt

### Issue 1
docs/sharing-the-cluster/db-branching/turbopuffer.md:26
When a user on the documented minimum CLI 3.260.0 or current CLI 3.261.0 copies this configuration, the CLI's `DatabaseBranchConfig` enum rejects `"type": "turbopuffer"` as an unknown variant, preventing the documented branch from being configured. Align this guide and its stated minimum version with a CLI release whose schema includes turbopuffer support.

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For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

Summary

The PR adds documentation and navigation for turbopuffer namespace branching, including configuration, copy modes, cleanup, limitations, and support-matrix entries. However, the documented configuration is not accepted by the current mirrord CLI schema.

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