we should explicitly control what goes in pkgdown references section

[jameslamb]

Right now, the references section looks like this:

I propose that we should:

• hide the Abstract* completely
• Put the two functions together in a section called "Functions"
• Put all reporters in a section called "Reporters"

See https://github.com/UptakeOpenSource/uptasticsearch/blob/master/r-pkg/_pkgdown.yml#L5 for how to customize this. It comes out looking like this: