Generated documentation for each branch as GitHub page

[dhebbeker]

I have adjusted (#41, #43) the GitHub workflows such that the documentation is generated for every push and deployed into a directory of the GitHub Page of the respective branch.

The GitHub Page is now a landing page where one can select for which branch the generated documentation shall be shown. This page could be beautified in the future.

This allows us to see the currently pushed documentation for every branch.

Note, that for some branches the generated documentation is missing. Those will be generated only until the enhanced workflows are merged into those branches.

[dhebbeker]

Problems with the documentation workflows were observed. I assume this was due to a race condition pushing to the repository. I have pushed a change (6c06330) in the hope of mitigating this issue: At least the two workflows pushing changes to the documentation should not be triggered simultaneously by the same push.

I guess there remains the chance, that two timely pushes may trigger workflows which then may collide.

Cc: @FiveTeethless