Implement bot to provide helpful comment on first contributions #1007

An idea that was brought up at out last community meeting is to create a bot (presumably through GitHub Actions) that would provide a helpful welcome message as a comment the first time that someone makes a pull request.

The welcome message should link to our development guide and provide information on or links about:

• Writing documentation (with links to guides on Sphinx and reStructuredText)
• Writing tests (with links to pytest, plus our test helpers when they're implemented)
• pre-commit
• A guide on git and GitHub
• The code review process
• Our communication channels and meetings
• Our code of conduct

We do want this to provide a warm welcome and an expression of gratitude, of course. We should also try to be succinct and not provide unnecessary detail. Thanks!