Make litedown vignettes compatible with pkgdown

[rikivillalba]

#6583
The purpose of this PR is to allow the vignettes to be rendered with litedown while still being compatible with rmarkdown since pkgdown uses rmarkdown to build the site. Some modifications were made:

• the {r} Sys.date() inline codes changed to !r Sys.date() (EDIT: this works in the YAML header only). They re compatible by both litedown and knitr/rmarkdown unlike r Sys.date() (knitr only) and {r} Sys.date() (litedown only)
• the script that adds the link to other language checks whether it is being rendered with knitr and if it is not the case, switch to litedown functions to get context.
• single (external) code block in each vignette/language to print the i18n message

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.08%. Comparing base (de27098) to head (1285908).

Additional details and impacted files

@@            Coverage Diff            @@
##           litedown    #7402   +/-   ##
=========================================
  Coverage     99.08%   99.08%           
=========================================
  Files            83       83           
  Lines         15800    15735   -65     
=========================================
- Hits          15655    15591   -64     
+ Misses          145      144    -1     

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[jangorecki]

I thought we have some workaround implemented for that in CI, but couldn't find it now. If there is we could possibly remove if this PR is merged.

[ChristianWia]

@rikivillalba ? you do not change the date with date: !r Sys.Date()
Same remark applies to the EN vignette.