Add example to documentation of each exported function

[frankfarach]

Required for rOpenSci packages: https://devguide.ropensci.org/building.html#general
See also: https://r-pkgs.org/man.html?q=example%20#man-functions

Add at least one example to the documentation for:
npi_search
npi_flatten
npi_summarize

Examples should not produce errors as they are run during R CMD Check.