Publish code coverage report with GitLab Pages

What this post added

This post details how to configure GitLab CI/CD to generate code coverage reports using tools like SimpleCov and then publish these reports as static websites using GitLab Pages. It provides specific `.gitlab-ci.yml` configurations for running tests, storing results as artifacts, and deploying them to GitLab Pages. It also touches upon parallelizing test suites and merging partial coverage reports, and how to add a code coverage badge to the README.md.