Create reasonable scenario for scripting tasks vignette

[zkamvar]

Problem

In #86, I migrated the scripting tasks configuration chapter of the user guide to here, but I and both of the reviewers noticed a problem: the structure of the config file doesn't really make much sense. It was merged in the interest of expedience, but now the task is to re-frame the narrative.

The most salient points that were brought up:

1. In add scripting tasks config vignette #86 (comment) and add scripting tasks config vignette #86 (comment), we noted that the model tasks that are created are not specific enough (they both include the same targets). This could be addressed by making the target in the task IDs unique and only including one target metadata in each task.
2. Related to the first point: we need a final section demonstrating adding a round and the consequence of accidentally creating an invalid config. I had initially authored this section, but removed it in 4889b55. I removed it because Anna brought up a very good point in add scripting tasks config vignette #86 (review): the specific example of adding another round in this case was unrealistic.

Potential solution

The solution does not need to be a brand new hub. In fact, pointing to a real-world example is a great way to strengthen mental models.

I think this might be an opportunity to reverse-engineer the flu sight hub as the grand narrative for part 1. That being said, it does not fit within the narrative to add another round object because the rounds are defined by variable and to add more rounds, the flu sight hub folks just added more dates to the task ID that defines the round ID.

Instead, for part 2, to demonstrate the addition of a round (and a demonstration of a failure when you include a duplicated round ID), we could walk through the script for the Variant Nowcast Hub.