[OSPO Book] Add a summary to each chapter

[alice-sowerby]

From @Pizza-Ria in #603

... add in a summary box (perhaps at the beginning of the chapter or the end that highlights 2-3 tips for quick consumption and orientation as to the value of the chapter ... you don't have to give everything away as we still want people to read the rest but a little teaser might be helpful).

This sounds sensible and useful.