Add some docs describing how to migrate from old volumes to new ones

[djeebus]

No description provided.

[cursor]

PR Summary

Low Risk
Documentation-only changes with no runtime or API impact.

Overview
Adds docs/volumes/migrate.mdx with JS/TS and Python examples that list volumes by name, connect or create the destination, mount both on a sandbox, install rsync, and run rsync -a from source to dest. Registers that page in the Volumes section of docs.json.

Adds Mintlify description frontmatter on the volumes overview and the manage, mount, read-write, info, upload, and download pages.

The volumes overview still lists manage, mount, read-write, info, upload, and download but does not link to the new migrate page, unlike the sidebar entry in docs.json.

The migration guide does not say that copying into an existing destination volume merges with current data or that mismatched volume types (only printed via JWT voltype) may be a problem.

^{Reviewed by Cursor Bugbot for commit 903f823. Bugbot is set up for automated code reviews on this repo. Configure here.}

[mintlify]

Preview deployment for your docs. Learn more about Mintlify Previews.

Project	Status	Preview	Updated (UTC)
e2b	🟢 Ready	View Preview	Jun 18, 2026, 2:19 AM

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 75d7427. Configure here.}