feat: Add config.yml for welcome bot

[isabelle-dr]

Summary:
Add a welcome bot for first time issues, first PR opened, and first PR merged. This is part of issue #1175.

Expected behavior:

The Welcome bot sends a custom message every time a new user opens an issue, a PR or gets a PR merged.

Please make sure these boxes are checked before submitting your pull request - thanks!

• Run the unit tests with gradle test to make sure you didn't break anything
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
• Include screenshot(s) showing how this pull request works and fixes the issue(s)

[isabelle-dr]

@maximearmstrong in order to test if it works before we merge, I added the config.yml file to my fork here.
Can you do the following on my fork:

• open an issue
• open a PR

Thanks! 🙏

[github-actions]

Thank you for this contribution! 🍰✨🦄

Information about source corruption

1 out of 1335 sources are corrupted.
The following sources are corrupted:

• de-unknown-ulmer-eisenbahnfreunde-gtfs-1081

Acceptance test details

The changes in this pull request did not trigger any new errors on known GTFS datasets from the MobilityDatabase.
Download the full acceptance test report for commit 4b367a9 here (report will disappear after 90 days).

[maximearmstrong]

Purrfect 😻 Thank you @isabelle-dr