Add job summary for coverage report

[pulsastrix]

Currently, the code coverage badge links nowhere.

If we had some kind of page that shows the current coverage report of this CI run even outside of PRs, we could link there instead.
This would also have the advantage that we could see the current coverage state on the current state of main.

Maybe we could create a job summary for the coverage report (see https://github.blog/news-insights/product-news/supercharging-github-actions-with-job-summaries/)?

Originally posted by @pulsastrix in #27 (comment)

[pulsastrix]

Done in #35