Add @keywords

[dsweber2]

per suggestion by @dajmcdon:

To view “all possible endpoints”, we could add @Keyword tags to each of their documentation files. This has 2 benefits. One is that this can be easily (and quickly) searched. So we could write a small helper, say, available_endpoints(), that would just list them all. If we add more later, we just remember to add the same keyword to the epicall function. The second benefit is that the _pkgdown.yml reference index can contain has_keyword(…), so we don’t need to list all the endpoints individually. Since this (and likely lots of other parts of the documentation) are shared across all signals, we could even extract anything shared to a single doc. I can probably do this as a PR if others think it would be worth having.

[dajmcdon]

Logging a pain point for posterity:

Despite adding @keyword endpoint in the Roxygen, these turn out not to create "keywords" in help.search(). The reason is that there is a list of standardized keywords and "endpoint" isn't among them. I believe that, because it's non-standard, and even though it's still converted to \keyword{endpoint} in the .Rd file, it actually gets stored as a "concept".

This was super annoying to try to figure out. You can view the standard keywords with file.show(file.path(R.home("doc"), "KEYWORDS"))

So, once #155 is merged, help.search(package = "epidatr", keyword = "endpoint") will find nothing, but help.search("endpoint", package = "epidatr", fields = "concept") will list all the endpoints.