Add stub CircleCI config.

[aaronweeden]

Description

This PR adds a stub configuration file for CircleCI. It will be filled out in a subsequent PR after setting it up in the CircleCI dashboard.

Motivation and Context

Looking to move from GitHub Actions to CircleCI for testing since we have higher usage limits there and to use the same tool as the other repos for easier management.

Type of change:

• Refactoring / documentation update (non-breaking change which does not change functionality)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Release preparation

Checklist:

• CHANGELOG.md has been updated
• xdmod_data/__version__.py has been updated to the next development version
• The milestone is set correctly on the pull request
• The appropriate labels have been added to the pull request
• Running the automated tests (see docs/developing.md) produces no errors
• Updates have been made to the xdmod-notebooks repository as necessary, and the notebooks all run successfully
• The changes in this PR have been ported/backported to other branches as needed