Support creating literature list sections at a specific index

This MR adds support for creating literature list sections at a specified index (/api/literature-lists/{id}/sections/{index}), extending the existing literature list section creation endpoint (/api/literature-lists/{id}/sections), as suggested by @aoelen. The provided index has to be a positive integer or zero. If the index is higher than the existing literature list section count, the new section will be appended at the end of the literature list, and no error will be thrown.