Add greet action 🎉 (#37499)

Summary:
Currently we do _not_ greet; **"First Time Contributors"**.
It is important to greet every first time contributor keep the OSS community thriving. 😊

This PR; Adds a greet action `greet.yml` :tada:, which comments under First Time Contributors PRs, greeting them as well suggesting some basic steps/guidelines.

## Changelog:

[GENERAL] [ADDED] - [Actions] Add greet action 🎉

Pull Request resolved: https://github.com/facebook/react-native/pull/37499

Test Plan: - Should greet 1st time contributors

Reviewed By: cortinico

Differential Revision: D46142126

Pulled By: cipolleschi

fbshipit-source-id: 5a619a485409fda6fe895feffb736e878fb206ee