Add GitHub welcome workflow for new contributors

[aniketpati1121]

Hi maintainers,

While contributing across Kubeflow repositories, I noticed that some repos (like kubeflow/sdk) already include a GitHub Actions–based welcome workflow to greet first-time contributors and guide them with helpful links.

The Kubeflow Pipelines repo currently does not have this workflow.

I would like to propose adding a minimal GitHub Actions workflow that:

• Automatically welcomes first-time contributors who open an issue or PR
• Provides links to the contributing guide and community channels
• Encourages participation and smooth onboarding for new contributors