GitHub Action: implement automation for welcoming new contributors

[kyleecodes]

Overview

Implement a GitHub Action that will automatically welcome new contributors in PRs.

Action Items

• Draft an informative message linking contributors to helpful information.
• Ensure that the action will not greet dependabot or any automated PRs.
• Write a new workflow and submit a PR.
• Document in internal docs.

Resources

https://github.com/marketplace/actions/auto-message-for-pr-s-and-issues
https://github.com/actions/first-interaction
https://github.com/marketplace/actions/first-interaction
https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#issue_comment