Add wiring diagram for SW SKR mini E3 V1.2

[GadgetAngel]

*updated Electrical Index.md file to add the Switch Wire SKR mini E3 V1.2. *added initial preparation diagram *added description for wiring diagram *added wiring diagram for Switch Wire *added the heatsink diagram to remind user to ensure that the heatsinks are installed on the stepper motor drivers *added Color Pin diagram for Bigtreetech SKR mini E3 V1.2 *added original SKR mini E3 V1.2 pinout *added URL to the original Bigtreetech Github Repo on the SKR mini E3 V1.2 board