docs: document building the template with build_prod.py

[mishushakov]

Documents how to build the E2B Code Interpreter template from this repo. Renames the customizing section to "Customizing the Code Interpreter template" and adds an explicit step for setting E2B_API_KEY in a .env file. Adds a new "Building the production template" section covering template/build_prod.py, installing requirements-dev.txt, and the SKIP_CACHE=true flag for clean rebuilds.

🤖 Generated with Claude Code

[cursor]

PR Summary

Low Risk
Markdown documentation changes only; no runtime or application code.

Overview
README adds a "Customizing the sandbox template" section that points to template/README.md for custom templates and production code-interpreter-v1 builds via build_prod.py.

template/README is retitled and split: new "Building the production template" covers requirements-dev.txt, .env with E2B_API_KEY, python build_prod.py, and SKIP_CACHE=true. The custom flow is under "Creating a custom template" with renumbered steps and an explicit .env step before building.

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