add section about dependency chains

[tavareshugo]

Started doing this in 06-job_dependencies.md. Ideas to do:

• [x] Schematic of a pipeline showing job dependency types (e.g. afterok and singleton).
• [x] Revise scripts used in the examples.
• [x] Design an exercise. Idea: for example give a schematic pipeline and ask to write mock scripts that recreate that chain of dependencies.