Add an automatic backlink for each chapter

Currently you can click on a timestamp in the list, and it jumps to a point in the notebook. It would be good if you could click on the anchor in the notebook, and it would jump to the corresponding timestamp. This is very similar to #8 (closed), but automatically.

Can we do the following: For all timestamps, if the timestamp has a jump target, and the jump target exists in the notebook, add some UI element to the notebook to jump to that timestamp in the video.

If the jump target is a heading, we can turn it into a link. Or we can add a little button next to it.

Additional considerations: We should not change the markdown source, because that messes up the build-in outline view of JupyterLab, among other things.