Add a TOC

Documentation/GCodes.md

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 ## Implemented G Codes
 
+- [Movement F and M Codes](#movement-g-and-m-codes)
+- [SD Card M Codes](#sd-card-m-codes)
+- [Hardware Control](#hardware-control)
+- [Temperature M Codes](#temperature-m-codes)
+- [Message M Codes](#message-m-codes)
+- [Endstops M Codes](#endstops-m-codes)
+- [Special Features M Codes](#special-features-m-codes)
+- [Units and Measures G and M Codes](#units-and-measures-g-and-m-codes)
+- [Firmware Retraction G and M Codes](#firmware-retraction-g-and-m-codes)
+- [Z Probe G and M Codes](#z-probe-g-and-m-codes)
+- [Filament Diameter M Codes](#filament-diameter-m-codes)
+- [EEPROM Settings M Codes](#eeprom-settings-m-codes)
+- [Delta M Codes](#delta-m-codes)
+- [Stepper Driver M Codes](#stepper-driver-m-codes)
+- [SCARA M Codes](#scara-m-codes)
+- [GCode Comments](#comments)
+
 ### Movement G and M Codes
 ```
 G0   -> G1