Demo collect_zoltar function in a vignette

[lshandross]

The collect_zoltar function should be demo'ed in a vignette somewhere, whether that's in the main vignette here in hubData or elsewhere, as noted in the comment from Nick below:

In terms of adding an example of this code to the vignette, I this this is not necessary for this PR because it adds complexity (e.g. storing secrets) and won't be reproducible for users without Zoltar accounts. But I will say that I large advantage of this function is how it will make the covid-19 forecast hub data directly available in hubverse format, and maybe giving a demo of that could be useful somewhere at some point. Maybe worth filing an issue here, or in the main hubverse repo, at some point about that.

Originally posted by @nickreich in #36 (review)