update navbar of homepage

[ben-schwen]

Adds the fread-and-fwrite vignette.
Adds the datatable_cheetsheat_pt_br cheatsheet
Updates the datatable_cheatsheet_fr pdf which seems to have a render error

[aitap]

The original source of cheatsheets is https://github.com/rstudio/cheatsheets, right?

[ben-schwen]

The original source of cheatsheets is https://github.com/rstudio/cheatsheets, right?

At some extend. The original one, including the underlying powerpoint, was submitted here AFAIR

[aitap]

I see, the French originals are from https://github.com/phgrosjean/rfrench/tree/main/data.table/cheatsheets (rstudio/cheatsheets#586).

[aitap]

The new link works fine when the website is rendered locally; the new cheatsheet is readable.