docs: Add Kubeflow migration announcement banner to README

[vara-bonthu]

🔄 Pull Request

📝 Description

Brief description of changes and motivation.

📢 Migration Announcement: Kubeflow Organization Approval

This PR adds a prominent announcement banner to the README informing users about the approved migration to the Kubeflow organization.

🎉 Background

• Proposal Status: Community proposal PR
  #873 has been officially approved by Kubeflow maintainers

• Migration Status: Planning phase - migration timeline to be tracked in repository issues

🎯 Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 📖 Documentation update
• 🧪 Test improvement
• 🔧 Refactoring (no functional changes)

🧪 Testing

• ✅ All existing tests pass (task test)
• 🔬 Tested with MCP Inspector
• 📊 Tested with sample Spark data
• 🚀 Tested with real Spark History Server (if applicable)

🔬 Test Commands Run

# Example:
# task test
# npx @modelcontextprotocol/inspector uv run -m spark_history_mcp.core.main

🛠️ New Tools Added (if applicable)

• Tool Name: new_tool_name
• Purpose: What it does
• Usage: Example parameters

📸 Screenshots (if applicable)

✅ Checklist

• 🔍 Code follows project style guidelines
• 🧪 Added tests for new functionality
• 📖 Updated documentation (README, TESTING.md, etc.)
• 🔧 Pre-commit hooks pass
• 📝 Added entry to CHANGELOG.md (if significant change)

📚 Related Issues

Fixes #(issue number)
Related to #(issue number)

🤔 Additional Context

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🎉 Thank you for contributing! Your effort helps make Spark monitoring more intelligent.