Added docs on Volumes

[mishushakov]

Note

Low Risk
Low risk: documentation-only additions plus a navigation update, with no runtime/code changes.

Overview
Adds a new Volumes documentation section (overview plus read/write, metadata, upload, and download pages) describing how to create/connect/mount/list/get info/destroy volumes and perform basic file operations.

Updates docs.json navigation to include the new Volumes group under the Documentation sidebar.

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