Keyboard: Add YMD96 instructions for Linux + Backlight Fixes (#3908)

Keyboard: Add YMD96 instructions for Linux + Backlight Fixes (#3908)

* Add flashing instructions for Linux

* Make device/manufacturer name consistent for ymd96

* Disable command mode by default for YMD96

* Add function call to fix backlight for YMD96

* Add personal YMD96 config

* Replace include with QMK macro

* Move KEYMAP macro to keymap-level folder

* Move custom KEYMAP macro to global LAYOUT_iso macro

* Change transparent keymappings to global definition

* Remove unecessary import and change to pragma once

 - (instead of repeated ifdef)

* Remove global keymap/layout definition