docs: add scale and HA sizing guide. index.md: extend.

[la-cc]

📝 Summary

Adds a new documentation page for kubara scale and HA sizing. It explains the main scale test findings, introduces S/M/L sizing guidance, and documents key Argo CD tuning and HA considerations

🧩 Type of change

• 🔧 CLI / Go code
• 📦 Helm chart
• 🧱 Terraform module
• 📝 Documentation
• 🧪 Test or CI change
• ♻️ Refactor / cleanup

⚠️ Is this a breaking change?

• Yes, this change breaks existing functionality (explain in summary)

🧪 Testing

• CI passed
• Manually tested (local/dev cluster)
• Unit tested
• Not tested (explain why below)

🔗 Related Issues / Tickets

✅ Checklist

• Code compiles and passes all tests
• Linting and style checks pass
• Comments added for complex logic
• Documentation updated (if applicable)

📎 Additional Context (optional)

Adds the scale and HA guide, overview image, and related presentation asset.