Atmega16U2/Atmega8U2 DFU articles refactor [HC-1599]

[seaxwi]

Combine Atmega16U2/Atmega8U2 DFU articles and document dfu-programmer for Windows.

[Renat0Ribeir0]

Left some suggestions for improving readability, typos, etc.

[Renat0Ribeir0]

Suggested change

	The firmware is included in the **Arduino AVR Boards** package. When it's installed, you can find the firmware inside the [Arduino15 folder](https://support.arduino.cc/hc/en-us/articles/360018448279-Where-can-I-find-the-Arduino15-folder-):
	* Windows: `%USERPROFILE%\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.8.6\firmwares\atmegaxxu2\arduino-usbserial`
	* macoS: `~/Library/Arduino15/packages/arduino/hardware/avr/1.8.6/firmwares/atmegaxxu2/arduino-usbserial`
	* Linux: `~/.arduino15/packages/arduino/hardware/avr/1.8.6/firmwares/atmegaxxu2/arduino-usbserial`
	You can also access the files from the [board package repository on GitHub](https://github.com/arduino/ArduinoCore-avr/tree/master/firmwares/atmegaxxu2/arduino-usbserial).
	Do one of the following:
	* Ensure that that **Arduino AVR Boards** package has been installed using Arduino IDE or Arduino CLI.
	* [Download the firmware for your board from GitHub](https://github.com/arduino/ArduinoCore-avr/tree/master/firmwares/atmegaxxu2/arduino-usbserial).
	Ensure the **Arduino AVR Boards** package is installed using the Arduino IDE or CLI. The firmware is included in this package and can be found in the [Arduino15 folder](https://support.arduino.cc/hc/en-us/articles/360018448279-Where-can-I-find-the-Arduino15-folder-) when it's installed:
	* Windows: `%USERPROFILE%\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.8.6\firmwares\atmegaxxu2\arduino-usbserial`
	* macoS: `~/Library/Arduino15/packages/arduino/hardware/avr/1.8.6/firmwares/atmegaxxu2/arduino-usbserial`
	* Linux: `~/.arduino15/packages/arduino/hardware/avr/1.8.6/firmwares/atmegaxxu2/arduino-usbserial`
	You can also download the firmware from the [board package repository on GitHub](https://github.com/arduino/ArduinoCore-avr/tree/master/firmwares/atmegaxxu2/arduino-usbserial).

Consider removing the "Do one of the following" instructions since they can be included in the steps before. Also, the alternative to download the firmware from github is provided already so maybe there's no need to suggest it again.

[Renat0Ribeir0]

this can be removed