samples: subsys: usb: mass: add support for Adafruit feather nRF52840

Add configuration and overlay files for Adafruit feather nRF52840.
README has also been updated and improved. Documentation related to
littlefs has been moved to an independent section as it could apply to
any sample.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>