Add vignette on creating a submission template

[zkamvar]

As mentioned in #162, we do not currently have instructions for submitting teams to use submission_tmpl() to create a submission template.

I have written a couple of examples in the Writing Model Output section in the hubverse documentation, but this is static and doesn't have the ability to show the whole process.

A vignette on creating model output might help modelers realize that they can use the infrastructure to help them create and validate submissions.