replace the current caching approach with cached markdowns

[idavydov]

Currently, we have some code to to cache slow parts of vignettes.

This code is ugly and relies on global variables, it also causes some warnings dring R check.

Instead we would like to put slow code into a child markdown document, which can be rendered manually when needed.

After discussing with @banfai we decided to put all the child documents (Rmds and mds) into vignettes/cached.

[banfai]

• [x] change output from md to html_fragment (output: html_fragment, this generates a self-contained HTML)
• [x] rename children to vignettes/cached/_*.Rmd (_something.Rmd are ignored by pkgdown, otherwise it renders everthing, even stuff in .Rbuildignore)
• [x] ~include vignettes/cached/~ in .Rbuildignore (so devtools::build_vignettes() ignore them) include vignettes/cached/.*\.Rmd, otherwise the compiled HTML fragments are not found
• [x] include ~knit: (\(input, ...) rmarkdown::render(input, output_file = "something.html"))~ in the yaml header knit: (\(input, ...) rmarkdown::render(input, output_dir = "vignettes/cached")
• [x] change {r, child="cached/_something.html"}
• [x] add !cached/*.html to vignettes/.gitignore

this seems to be working with both devtools and pkgdown

[idavydov]

Congrats! 🎉