Develop documentation on how to use sjlabelled

Related to #290

The qualtRics package adds labels to surveys that can be used with sjlabelled but we don't surface this very clearly. How can we better communicate this to folks? We could:

• keep going at the end of this vignette and show how to use sjlabelled on the newly fetched survey
• add a section on add_var_labels to the fetch_survey() docs with a code chunk like we have for include_questions

A challenge here is that none of the documentation actually queries Qualtrics live so we can't easily show the output anywhere. One option would be to save an example survey (output of fetch_survey()) as a data object in the package to use in examples and vignettes?