Added docs on Volumes

[mishushakov]

Note

Low Risk
Low risk documentation-only change that adds new pages and updates sidebar navigation, with no runtime code or API behavior changes.

Overview
Adds a new Volumes documentation section, including overview and how-to pages for managing volumes, mounting them to sandboxes, and performing file operations (read/write, metadata, upload, download) with JS/TS and Python examples.

Updates docs.json navigation to include the new docs/volumes/* pages in the sidebar.

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