Add coverage report to documentation

[JulienDoerner]

With this PR the coverage report is added to the documentation.
The Github action is adopted to copy the report into the documentation part and a link to the files is added.

In case of a manual build of the documentation the coverage report has to be copied by hand to the right location. In the build folder it is done by:

cp -r coverageReport doc/pages/coverageReport

[lukasmerten]

If we are on the automatic creation of the documentaion, we could also think about automatic deployment to gh-pages.

- name: deploy documentation #deploys the documentation to the gh-pages branch
uses: JamesIves/github-pages-deploy-action@v4
with:
folder: ./build/doc

Following this description here: https://github.com/JamesIves/github-pages-deploy-action

In case this seems to be useful, I'll can either provide a new PR or just add to this one.

[JulienDoerner]

I will add the deployment in this PR. For the testing phase I will convert it to draft and notify you when it is ready.

[JulienDoerner]

I changed the workflows accordingly.
Now the steps are seperated in three different task

1. The create_documentation builds the docu and uploads the artifact. The coverage report is only included in the documentation and not seperatly stored.
2. The create_coverage_report builds the coverage report and uploads it as an artifact. This can be used to check the changes with new PRs.
3. The deploy_new_documentation workflow builds the new documentation and push it automaticly to the gh-pages branch. This works in my fork, but we might have to check the permissions. It is configured to run in all cases, where we push to the master branch. This should be done as the coverage report and the API information are changed with all changes on the master branch.

[lukasmerten]

Coverage Report

[lukasmerten]

The most recent coverage report is linked here.