SD formatting guide: SD Association's SD Card Formatter is now available for Linux

[mariomadproductions]

SD Association released a Linux version: https://www.sdcard.org/downloads/sd-memory-card-formatter-for-linux/
So this should probably be integrated into the Linux section of SD formatting guide, like it is for Windows/Mac (maybe leaving the built-in Linux tools as a alternative option).

[m4x10187]

I've made a pull request with the instructions.