Use coveralls instead of static coverage report

I would suggest to remove the static html page from the build, make gh pages host only docs (if not completely dropped in favour of RTD), and use coveralls for (fancy) coverage reports.

Look at the project's build configuration to find where the static HTML coverage report is generated. Research how to integrate Coveralls for coverage reporting in a Python/Jupyter project. Determine if GitHub Pages is currently used for docs and if ReadTheDocs (RTD) is a better alternative. The goal is to replace the static report with Coveralls and adjust the CI/CD pipeline accordingly.

索引モデルが issue の本文から書いたものです。