add existing datatable_fr to cheatsheets/translation/french #586
Description
Refer to #367 and Rdatatable/data.table#6455 (comment) to add existing cheatsheet at the right place :
- retrieve initial works pdf and pptx from directory -> https://github.com/phgrosjean/rfrench/blob/main/data.table/cheatsheets/data_table-fr.pdf
- rename into datatable_fr
- upload in translations/french to officialise first attempt
- check then the current EN version of datatable.pdf in ./cheatsheets
- report possible updates from EN to FR
- commit and request a pull in rstudio/cheatsheets