translated vignette proposal : all pages link back to EN root vignette

[ChristianWia]

The EN vignette holds links towards translated pages (for example https://rdatatable.gitlab.io/data.table/articles/datatable-intro.html leads to FR transltion in -> https://rdatatable.gitlab.io/data.table/articles/fr/datatable-intro.html

In the translated page, there is no longer useful to reference the page itself, nor all other existing translations (for page maintenance reasons when new language is added)

So my proposal would be that all translated pages display the same equivalent of :

Translations of this document is available in

English

[rikivillalba]

Some chunk could be provided that generate the link list, provided the corresponding files and directories exist
I.e.

```{r echo = FALSE}
  url = "https://rdatatable.gitlab.io/data.table/articles"
  path = dirname(knitr::current_input(TRUE))
  if (basename(path) == "vignettes") {
    lang = "en"
  } else {
    lang = basename(path)
    path = dirname(path)
  }

  translation = dir(path,
    recursive = TRUE,
    pattern = glob2rx(knitr::current_input(FALSE))
  )
  transl_lang = ifelse(dirname(translation) == ".", "en", dirname(translation))

  block = sprintf("%s\n", paste(sprintf(
    " [%s](%s) ",
    transl_lang[transl_lang != lang],
    file.path(url, sub("(?i)\\.Rmd$", ".html", translation[transl_lang != lang]))
  )))

  knitr::asis_output(block)
``` #

[rikivillalba]

Relevant: #6455

[tdhock]

the idea of automatically generated links between languages is interesting, but where would you put that code? (to avoid duplicated logic in each vignette)

[rikivillalba]

#6618 is my proposal