Show how to break an ol (#856)

* Show how to break an ol

Close #750

The theme uses CSS counters for ordered lists.
So to continue the numbering of an ordered list after a break,
use `style="counter-reset:none"` instead of `start="42"`.
Add the example from #750 to `docs/index-test.md` to test.

* Show how to start from 42

Close #750

The theme uses CSS counters for ordered lists.
To start the numbering of an ordered list with 42,
use `{:style="counter-reset:step-counter 41"}` instead of `start="42"`.

Add the example from #750 to `docs/index-test.md` to test.