update Automated testing documentation section

[elmjag]

Update documentation on CI testing to talk about the GitHub Actions.

Replace text on Azure Pipelines with one on GitHub Actions. I'm guessing that Azure Pipelines have been replaced by GitHub Actions at some point as I don't see the azure-pipelines.yml file in the repository anymore.

Thanks for contributing, make sure you address all the checklists (for details on how see development documentation)

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation