Provide more comprehensive documentation for building arduino-usbserial firmware

### Describe the request

Expand the instructions for building the repository's arduino-usbserial firmware to provide all information needed to ensure a successful build by anyone.

### Describe the current behavior

I'm trying to build the arduino-usbserial firmware to include custom features. There are instructions for building the firmware:

https://github.com/arduino/ArduinoCore-avr/blob/master/firmwares/atmegaxxu2/arduino-usbserial/readme.txt

However, I find them to be somewhat vague and I have not had any success following them.

### ArduinoCore-avr version

c8c514c9a19602542bc32c7033f48fecbbda4401

### Additional context

It would be helpful if the specific version of LUFA verified to be compatible with the firmware code was specified.

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Not directly related to the documentation request, but it would also be a good idea to set up a GitHub Actions workflow that builds the firmware on any change to the files, and periodically. This would validate the functionality of the intended build procedure, which might be broken by changes to the firmware or by external changes to the build dependencies.

#### Related

- https://github.com/arduino/ArduinoCore-avr/issues/501

Start with firmwares/atmegaxxu2/arduino-usbserial/readme.txt and the arduino-usbserial firmware sources. Reproduce the documented build procedure, record the required tools, dependencies, and compatible LUFA version, then update the instructions so another user can build successfully from a clean setup.

Written by the indexing model from the issue text.