Add a welcome bot

[Ankita297]

Is your feature request related to a problem? Please describe.

A github action to greet contributor on opening his/her first PR or first issue .

Describe the solution you'd like

• A clear and concise description of what you want to happen.

Describe alternatives you've considered

• A clear and concise description of any alternative solutions or features you've considered.

Additional context

• Add any other context or screenshots about the feature request here.

[sakshikhachane]

@Ankita297 Go ahead!