docs: index sandbox extension runners and tutorials in examples

[comp]

Summary

The sandbox section of the examples index (docs/examples.md) was missing a large number of examples that already exist under examples/sandbox/, so readers browsing the documentation had no way to discover them. This adds index entries for:

• The third-party sandbox provider runners under examples/sandbox/extensions/ — Daytona, E2B, Modal, Vercel, Cloudflare, Blaxel, and Runloop.
• Durable Temporal sandbox workflows.
• The tutorials suite under examples/sandbox/tutorials/ — dataroom Q&A, structured financial metric extraction, repository code review, sandbox resume, and vision-based website cloning.

Descriptions are drawn from each example's module docstring. There are no source or behavior changes.

Test plan

Docs-only change (markdown additions to an existing section of the examples index). Per AGENTS.md, docs-only changes are exempt from the $code-change-verification stack.

• Verified that every one of the 13 newly referenced example paths resolves to an existing file in the repository.
• No changes to mkdocs.yml navigation or generated reference files; the edit only appends list items to the existing sandbox section.

Issue number

N/A

Checks

• I've added new tests, if relevant — N/A (docs-only)
• I've run .agents/skills/code-change-verification/scripts/run.sh — N/A (docs-only, exempt per AGENTS.md)
• I've confirmed all verification steps pass (all referenced example paths verified to exist)
• If using Codex, I've run /review before submitting this PR — N/A

[seratch]

Thanks for the suggestion, but we won't list all the files under the directory for now.