Fix pkgdown configuration for reference

[maelle]

Some topics are missing from the configuration file.

-- Building function reference -------------------------------------------------
Error in check_missing_topics(rows, pkg) : 
  All topics must be included in reference index
• Missing topics: NLMR-package

Note that for topics you do not want to include in the index you can create an "internal" section https://pkgdown.r-lib.org/reference/build_reference.html?q=internal#missing-topics

To check all topics are listed, after editing the configuration file you can run pkgdown::check_pkgdown().

[maelle]

@marcosci @bitbacchus (in case you missed this, not that urgent :sweat_smile: )

[mhesselbarth]

Should be fixed in #101 - please check and merge