Add local development setup guide for whole system

[andreahlert]

Tracking issue

Closes #6839

Why are the changes needed?

The v2 branch has setup docs scattered across individual services (executor/DEVELOPMENT.md, dataproxy/DEVELOPMENT.md, manager/README.md) but there is no single, unified guide for getting the whole system running locally. Contributors need to piece together instructions from multiple files and the PR #6840 comment thread.

What changes were proposed in this pull request?

Add a root-level DEVELOPMENT.md with step-by-step instructions covering the full local development workflow using the unified manager binary:

• Prerequisites - Go 1.24+, Docker, kind, kubectl
• Quick Start - 7 numbered steps from cluster creation to health check verification
• Teardown - Clean shutdown and cluster deletion
• Individual Services - How to run each service standalone for focused development
• Configuration - Key config settings with reference to full config file
• SDK Testing - How to connect flyte-sdk and run workflows
• Useful Commands - Common kubectl, sqlite3, and make commands
• Troubleshooting - Solutions for common issues (kubeconfig, ports, MinIO, migrations)
• Architecture diagram - Visual overview of the system

How was this patch tested?

• Verified all referenced file paths exist in the v2 branch
• Verified all referenced Make targets exist
• Cross-referenced with the setup steps from PR [Feat] add data proxy service #6840 comment and existing service-level docs

Labels

• added

• main
  • Flyte 2 WIP #6583
    • Add local development setup guide for whole system #6900 👈

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[andreahlert]

Hi @pingsutw @machichima, could you take a look when you have a chance?

I put together a root-level DEVELOPMENT.md covering the full local setup for the unified manager binary, based on the steps from the #6840 comment. It consolidates the scattered docs from executor/, dataproxy/, and manager/ into a single guide with prerequisites, quick start, teardown, troubleshooting, etc.

Happy to adjust anything based on your feedback.