docs: add sandbox examples to examples index

[saime428]

Summary:

• Add the sandbox examples category to the examples documentation index.
• Link to the main sandbox examples README and representative sandbox example files.
• Include sandbox tutorials in the examples overview.

Why:

• The repository has a populated examples/sandbox directory, but the examples index did not list it.
• This makes sandbox agent examples easier to discover from the docs.

Validation:

• git diff --check
• Verified each linked examples/sandbox path exists locally.
• uv run mkdocs build --strict --site-dir <temp> did not pass because the current docs tree emits existing strict-mode warnings unrelated to this change.

Notes for reviewers:

• Docs-only change; no runtime code, dependency, or lockfile changes.