Document joint input design

[infotroph]

• Docs should explain what it should look like, what information it contains, how it is used
• mention in modules/uncertainty/NEWS.md (one sentence with relevant PR numbers)
• Mention in CHANGELOG.md (short but enough detail for users to know why it's important and what they'll need to change to use it in their workflows)

[dlebauer]

will be fixed by #3660 once changes from divine7022#1 are included

[infotroph]

Also: Update examples /demos to show an explicit call to generate_join_input_design as part of the workflow script rather than hiding it inside runModule_run_write_configs (OK if this doesn't go into current release)