docs: Note 'config' object shouldn't be accessed after initial load

Update Code_Overview.md to note that the config object should not be
stored after the "config loading phase".

Remove a few inadvertent cases where a 'config' object was stored
in module member variables.

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

klippy/extras/z_thermal_adjust.py

@@ -16,7 +16,6 @@ class ZThermalAdjuster:
         self.printer = config.get_printer()
         self.gcode = self.printer.lookup_object('gcode')
         self.lock = threading.Lock()
-        self.config = config
 
         # Get config parameters, convert to SI units where necessary
         self.temp_coeff = config.getfloat('temp_coeff', minval=-1, maxval=1,