Add examples for using `scale = vec` arguments for weighting of observations

[njtierney]

Documentation currently says:

How to allow weighting of observations? To weight your observations, say when using empirical logit, you must provide a vector of variances, for each observation, which represents the uncertainty of measurement. You can do this, provided the family = "gaussian", add the argument, scale = vec, to fit_hts, where vec is a vector of variances for each observation.

But I don't provide any worked examples of how to do this. I think the scale term belongs in the ... list at the end of the fit_hts function.

e.g.,

m <- fit_hts(
formula = pr ~ avg_lower_age + hts(who_subregion, country),
.data = malaria_africa_ts,
family = "gaussian",
special_index = month_num,
list(scale = vec)
)