Teacher-targeted documentation

Many of the teacher-facing features of CodeWorld are not well documented. This includes:

1. The requirements checker, useful for additional feedback.
2. Import by hash, useful for designed exercises with domain-specific framework or scaffolding.
3. The CodeWorld.Image module, useful for more easily importing educational assets.
4. The CodeWorld.Parameter module (#1243), useful for easy interactive student exploration.
5. Clipping (useful for incorporating student images into bigger context without overflow).
6. Prelude replacements, when available (#1245)
7. Embeddable environments, when available (#1248)