ARRAY_BY_EXTRUDERS, shorthand to sync planner

- Add some documentation to planner and stepper headers
- Patch up RAMBO pins with undefs
- Add `sync_plan_position` inline to set current XYZE
- Swap indices in `extruder_offset` to fix initialization values

Marlin/stepper.cpp

@@ -1205,7 +1205,7 @@ void microstep_init() {
     pinMode(E0_MS1_PIN,OUTPUT);
     pinMode(E0_MS2_PIN,OUTPUT);
     const uint8_t microstep_modes[] = MICROSTEP_MODES;
-    for (int i = 0; i < sizeof(microstep_modes) / sizeof(microstep_modes[0]); i++)
+    for (uint16_t i = 0; i < sizeof(microstep_modes) / sizeof(microstep_modes[0]); i++)
       microstep_mode(i, microstep_modes[i]);
   #endif
 }