todos for conditional workflow documentation

From https://github.com/common-workflow-language/user_guide/pull/189

- [ ] A follow up section indicating the use of pickValue to implement the patterns outlined in the proposal common-workflow-language/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`

https://github.com/common-workflow-language/user_guide/pull/189#pullrequestreview-469500443