Better table of contents layout

[edemaine]

Tomohiro writes: "The vertical line is more understandable if it is associated with the parent, but currently the line is connected to the left margin of the parent line. Is it possible to remove the left margin?"

• Stop vertical bar a little short, to match padding of text to the right. This makes it clearer where the subtree ends.
• Remove left margin of table of contents entries
• Increase gap after vertical lines from 3px to 5px
• Experiment with different purple colors for distinction
• Also add overflow-wrap to avoid horizontal scrollbar