Documentation: enable automatic table of contents

The default style was changed to disable floating, because it does not
work when the TOC is floated next to a math element: the mathjax element
spans the whole screen and makes the links in the TOC unclickable. There
is some handle-floats extension but it does not work as advertised:

https://docs.mathjax.org/en/v2.7-latest/options/extensions/handle-floats.html