Enable documentation of non-function package elements

[apreshill]

Hi @hadley,

As we discussed at the R Markdown team meeting, we have a number of non-function package elements that we'd like to be able to document including:

• add-ins,

• R Markdown templates (usually used in combination with rmarkdown::draft()),

• Project templates (often have an associated function, but sometimes not),

• learnr tutorials, and

• connections.

cc @cderv @jjallaire @yihui

It'd be great to be able to document these within the packages themselves using roxygen2, and for that to filter down into pkgdown sites too.

[gadenbuie]

Along those lines, it could be useful to document knitr chunk options, such as those provided by learnr. Perhaps in tandem with a formal IDE method for registering these new chunk options: rstudio/rstudio#9037.

[hadley]

Another important case — documenting roxygen2 tags! And there @lionel-'s experiment there suggests a pkgdown connection — currently the index page displays functions from the usage or the topic name; there's currently no way to list multiple aliases that might be informative to the user.

[hadley]

I think now we'd say you should just organise these into vignettes or meta-Rd files; unfortunately I don't think this will ever get to the top of the to do list for roxygen2.