add existing datatable_fr to cheatsheets/translation/french

Refer to #367 and https://github.com/Rdatatable/data.table/pull/6455#issuecomment-2327977767 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