Estimated reading time for each section and the whole tutorial

[seisman]

The sidebar shows a very long list of sections, which looks scary to readers, although each section may be very short.

I feel that it's useful to show the estimated reading time for each section and the whole tutorial. I think we should limit the estimated "reading" or "learning" time of the whole tutorial to less than 3 days.

[core-man]

We may test it when we finish it or some sections.