feat: add greetings workflow for contributors

[harmeetsingh11]

Related Issue

Closes #138

Description:
This pull request introduces a GitHub Action workflow that automatically greets contributors when they create new pull requests or issues. The goal is to enhance the project's community engagement and foster a welcoming environment for contributors.

Changes:

• Added a new .github/workflows/greetings.yml file for the greetings workflow.
• Configured the workflow to trigger on new pull requests and issues.
• Included a customized greeting message to acknowledge contributors.

To run this workflow you need to enable the following permission:

1. Go to your project repository settings
2. On left pane > Actions > General

3. Scroll down and Go to Workflow permissions > select "Read and write permissions"

4. Click save and its done.

Checklist

• My code adheres to the established style guidelines of the project.
• I have included comments in areas that may be difficult to understand.
• My changes have not introduced any new warnings.
• I have conducted a self-review of my code.

@shelcia please review this PR.
Thanks

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[harmeetsingh11]

@shelcia can you please add iwoc24 and difficulty level tag to the PR? Otherwise, my points won't add.