Content files restructure for compatibility with Hugo

The hugo migration will become significantly less hacky if we bring the current content into a hierarchical format that reflects the page structure.
Moving them around in jekyll should not be an issue, as we use permalinks in all pages.

To use the navigation features, we should use the navigation hierarchy as the content structure with folders:

- quickstart
- docs
- tutorials
- community
- about

IIRC some things cannot be directly mapped to the hierarchical structure, such as https://precice.org/docs.html which should be docs/index.html but really is docs/fundamentals/index.html.

Furthermore, some sections in the docs don't have an overview page.

Finally, it is still unclear to me how to handle the nested sections such as https://precice.org/configuration-mapping.html which should be docs/configuration/basics/mapping. Such subsections may need some overview pages too.