Make use of GitHub's default anchors.

I've removed the custom `<a name=''>..</a>` tags and the TOC is now relying on the generated anchors, so that not the full heading but only the anchor in front of it is clickable. Which makes the page calmer.

Also updated the back to top link from **[[⬆]](#TOC)** to **[⬆ back to top](#table-of-contents)** and repositioned them to not be part of the generated list item.