add items for help pages

[hturner]

Considered but not included:

• inlinedocs - seems only to be used by one of author's own packages, superseded/out-competed by roxygen2.
• gbRd - mainly a workhorse package for Rdpack, already mentioned.
• katex - has math_md() function for embedding math in R documentation; can't see this is any different from what R does since R 4.2.0. Similarly mathjaxr may now be superseded by improved KaTeX/MathJax handling in Rd files. katex may be worth mentioning in the vignette section for creating HTML vignettes where math continues to display offline (need to test).

[llrs]

Nice catch about tools::checkRd! It looks great with lots of useful packages.

I am not sure if in text roxygen2 comments should be that or just roxygen comments (I started converting the later to the first but stopped as I saw it was consistent). The first matches the package name which will help people new to it, the second one is easier to read but might confuse readers.

[llrs]

About the katex and mathjaxr, if they are no longer recommended it does not make sense but katex might still have some uses.

[hturner]

About the katex and mathjaxr, if they are no longer recommended it does not make sense but katex might still have some uses.

I think katex was mainly a package that filled a gap while base R and other packages caught up. E.g. here is Christophe Dervieux discussing it in the context of rmarkdown/bookdown: rstudio/bookdown#401 (comment). So I think we can leave it out and reconsider if someone makes a case for it.

[hturner]

Discovered CRAN package {Rd2md} by looking at {ropenscilabs/r2readthedocs}

[llrs]

Nice, I would merge it as is. Thanks!