[WIP] Add GitHub action: Greetings.yml - Automates advice to JabRefs first time code contributors

[ThiloteE]

Introduces a GitHub action that automatically comments and gives advice to first time contributors of the JabRef repository. Only triggers upon assignment of users to an issue.

Relevant documentation:

• https://github.com/actions/first-interaction
• https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#issues
• https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#onevent_nametypes

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.