Move Lua filter vignettes from rmarkdown into a book chapter

[cderv]

From https://github.com/rstudio/rmarkdown/pull/2108#issuecomment-819806700

we should add a part about Lua filters in this book based on the vignette: https://pkgs.rstudio.com/rmarkdown/articles/lua-filters.html

Intro and examples about Lua filters are in the Cookbook: https://bookdown.org/yihui/rmarkdown-cookbook/lua-filters.html