[Tooling] Add GH Action to Greet New Contributors and Explain Submission Guidlines.

[BethanyG]

Similar to this one from Elixir (and probably based off it....)

https://github.com/exercism/elixir/blob/main/.github/pr-commenter.yml

[BethanyG]

Just commenting here to track progress.

PR 2684 does auto-commenting for new issues.