Indents for contents

[sherdim]

Request for a new feature

as marius311 noted on earlier stages:

• With numbering turned off, I find it difficult to parse the nested level of the headings. The numbering gives them a natural indent, but without the numbering there's no indentation, and the fact that the collapser arrow is the same size and position independent of nesting level kind of causes some visual confusion for me. Just as an example:
  

Originally posted by @marius311 in #34 (comment)

The decision of Jupyter TOC2 extesions is OK
https://jupyter-contrib-nbextensions.readthedocs.io/en/latest/nbextensions/toc2/README.html

[sherdim]

Sorry, it is a double of #144