manual_probe: Add a helper script for performing manual Z probing

Add MANUAL_PROBE and Z_ENDSTOP_CALIBRATE commands.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/G-Codes.md

@@ -118,6 +118,23 @@ The following standard commands are supported:
 - `STEPPER_BUZZ STEPPER=<config_name>`: Move the given stepper forward
   one mm and then backward one mm, repeated 10 times. This is a
   diagnostic tool to help verify stepper connectivity.
+- `MANUAL_PROBE [SPEED=<speed>]`: Run a helper script useful for
+  measuring the height of the nozzle at a given location. If SPEED is
+  specified, it sets the speed of TESTZ commands (the default is
+  5mm/s). During a manual probe, the following additional commands are
+  available:
+  - `ACCEPT`: This command accepts the current Z position and
+  concludes the manual probing tool.
+  - `ABORT`: This command terminates the manual probing tool.
+  - `TESTZ Z=<value>`: This command moves the nozzle up or down by the
+    amount specified in "value". For example, `TESTZ Z=-.1` would move
+    the nozzle down .1mm while `TESTZ Z=.1` would move the nozzle up
+    .1mm. The value may also be `+`, `-`, `++`, or `--` to move the
+    nozzle up or down an amount relative to previous attempts.
+- `Z_ENDSTOP_CALIBRATE [SPEED=<speed>]`: Run a helper script useful
+  for calibrating a Z position_endstop config setting. See the
+  MANUAL_PROBE command for details on the parameters and the
+  additional commands available while the tool is active.
 - `RESTART`: This will cause the host software to reload its config
   and perform an internal reset. This command will not clear error
   state from the micro-controller (see FIRMWARE_RESTART) nor will it