Switch to git subtree

[koppor]

We have issues with git submodules. Since we "just" need to have the code in our repostiory, git submoulde seems to be the better choice.

This PR switches from sub modules to sub trees. Including an update twice a month. Maybe, I need to play around with push-rights to the master branch. Will do that later to keep things moving.

Sorry for 6408457. This should not be in here, but it's difficult to remove it with git rebase -i.

More background information:

• https://codewinsarguments.co/2016/05/01/git-submodules-vs-git-subtrees/
• https://svij.org/blog/2019/11/10/git-subtree-eine-alternative-fur-git-submodule/

[Siedlerchr]

I would suggest you create an ADR as well for this decision and please update the getting started guides on cloning and fetching
Otherwise subtrees seem to be a better solution.

[tobiasdiez]

LGTM. A follow-up would be to write a small python script that creates and updates an index file for the citation styles (and then also include this in the update workflow)