Add developer documentation for new compositional pipeline code

- [ ] Video
- [ ] Developer overview in contributor docs
- [ ] Diagram
- [ ] Docstrings on things like `Target`, `BaseTarget`, etc
- [ ] Add document on difference between weak, raw, plain, and token targets
- [ ] Glossary, including at least:
  - [ ] domain
  - [ ] removalRange
  - [ ] contentRange
  - [ ] iterationScope
  - [ ] boundary
  - [ ] interior
  - [ ] target