Use case-specific documentation pages

[leeper]

It would be helpful to have a set of pages showing how to use the package for specific use cases that are not tied to any specific function.

• Add use cases Rd documentation pages
  • Surveys, including panels ("survey", "survey panel", "survey experiment")
  • Categorization/moderation task ("categorization", "content moderation")
  • Sentiment rating ("sentiment", "sentiment analysis")
  • Web scraping task ("webscraping")
• Link to these from the main package documentation Rd file

These need to use the \alias{} and \concept{} macros in the Rd files