Document customizing the sandbox template

[mishushakov]

Mirrors the documentation added to code-interpreter#289 for the Desktop repo. Expands template/README.md into a full guide with a "Building the production template" section (poetry install, set E2B_API_KEY in .env, poetry run python build_prod.py) and a step-by-step "Creating a custom template" section that uses from_template("desktop") and shows usage via both the Python and JS desktop SDKs. Also adds a "Customizing the sandbox template" section to the main README.md that links to the template guide.

🤖 Generated with Claude Code

[cursor]

PR Summary

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

Overview
The root README adds a short Customizing the sandbox template section that points readers to template/README.md.

template/README.md is expanded from a minimal build note into a full guide: how to build the official desktop template with Poetry, .env, and build_prod.py (plus build_dev.py for dev), and how to create a custom template via Template().from_template("desktop"), a build.py example, and Python/JS Sandbox.create usage with a custom alias.

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[tomassrnka]

lgtm