todos for conditional workflow documentation

[mr-c]

From #189

• A follow up section indicating the use of pickValue to implement the patterns outlined in the proposal Proposal: conditionals II (refinement of run_if) common-workflow-language#854
• Add the example code and foo.cwl to https://github.com/common-workflow-language/user_guide/tree/gh-pages/_includes/cwl so that readers can run the example locally and we can test with CI like we do the others
• Add example of "extra" input used only for the when

#189 (review)