Initial support for CI

[ruda]

Create the initial CI by means of GitHub Actions / Workflows.

References:

• https://docs.github.com/en/actions/quickstart
• https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners
• https://github.com/actions/virtual-environments/blob/main/images/macos/macos-11-Readme.md
• https://github.com/actions/virtual-environments
• https://github.com/actions/starter-workflows/blob/main/ci/django.yml