Reworked backend structure to allow for a better separation

This PR aims to rework the backend in order to make it more usable with multiple content repositories. In order to do that, more functionality is moved to the content repository, while this backend is the rough guidance.

Changes

Moved CI/CD Pipeline to content repo

The idea is that each content repository can now have it's own pipeline, so that they can build their pages and upload it to different locations.

Added a Modulhandbuch-Theme

Themes in hugo are similar to the layouts folder that is currently in use. The difference is that a layouts folder can overwrite parts of the theme folder if necessary.

Therefore, the basic structure and css are in the theme folder in the backend, while the content specific layouts, such as the index.html with the clickable overview are now relocated to the content repository.
With this change, content repos can have their own overview on the start page.

Removed git submodules

With the submodule approach, changes to the content repo also had to be commited to the backend repo. With the manual cloning of the backend repo, submodules are no longer necessary