Resolve inconsistent use of headers

[wenkokke]

For screen readers, LaTeX and EPUB, and many other ways to read the book that aren’t simply a live website, it is important that headers are used consistently, i.e., starting at h1 and increasing by one header level at a time.

Our current convention always uses h4 for exercises, which leads to inconsistent increases, often jumping from h2 to h4.

If there’s no current use of h3, we could simply lift all exercises. Otherwise, we may have to find another way of marking exercises, without using headers—at least in the generated HTML.

If we need to systematically rewrite these, we can rely on—and codify—the convention that “#### Exercise” is always used for exercises.