Fix some issues with context handling at deployment time

[jemrobinson]

✅ Checklist

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⤴️ Summary

• Add documentation about how to add a new SRE
• Enable dsh config show to dump to a file
• Raise exception if requested to deploy an SRE that does not have a config yet
• Add Pydantic template for SREs
• Add validation for SRE index (0-256 and unique)
• Fix pytest exception changes

🌂 Related issues

See #1709

🔬 Tests

Tested on new SHM deployment

[JimMadge]

Looking good 🎉.

Just a few questions.

Thanks for fixing all of my pytest.raise mistakes 🙏.