Make page per section configuration less confusing

We have quite a lot of issues related to sections configuration, especially the new sectionDepth option. It seems hard to understand (and honestly I don't understand it too).

@okonet @bebraw @rafaesc How can we improve this API? It would probably make more sense to have an option on each section that will define whether this section should be rendered with all subsections or not. But I don't have enough experience with pagePerSection option to say much.

Related issues: #1139 #1137 #1058