Redo keep-together markup

- Avoid splitting headers from their introductory paragraphs.
- All closing div tags need a blank line before (lacking this, images
  display as inline instead of block, so text flows around them).
- Let text and equations split across pages (treat equations as text).
- Fixed existing cases with extra or no closing div tags.

Changes reviewed in print for all three books.

Resolves RayTracing#944

Author: hollasch