everything() helper for including missing references

[DanChaltiel]

I'm building my _pkgdown.yml file and I think it would be great to be able to include all missing references with some helper, such as everything().

I would like to be able to write something like this:

reference:
- title: "Main functions"
  desc: "I really have something to say about these 2."
- contents: 
  - fun1
  - fun2
- title: "Misc"
  desc: "Other minor functions"
- contents:
  - everything()

There is a 2 years old SO question about that here, but the solution is not that simple.

I'm a bit surprised that I couldn't find this mentioned anywhere, sorry for the inconvenience if my research was incomplete and this is a duplicate.

[SangdonLim]

I would find this feature useful.
Maybe adding a new argument to build_ fuctions like build_site(unique = TRUE) is a way to do it in FP.

[llrs]

I came here to suggest something similar but I think it is enough with a to have a function that reports functions not on _pkgdown.yml to the user. I think that if the check_missing_topics could be exported and it would be easier for authors to solve this

[hadley]

I think I'd prefer to resolve this through something like #1378, so that you easily find out when topics are missing.

[llrs]

While that PR solved how to make the user aware, I think that having some exported function would still have value (for instance on the typical checklist of releases there is an explicit mention to this). Would you consider a PR exporting check_missing_topics function?