Update rocket.add_parachute to use radius=None for consistency

Changed the default radius from 1.5 to None in the add_parachute method
to match the Parachute class behavior. This ensures consistent automatic
radius calculation from cd_s across both APIs.

Co-authored-by: Gui-FernandesBR <63590233+Gui-FernandesBR@users.noreply.github.com>

Author: Copilot

rocketpy/rocket/rocket.py

@@ -1475,7 +1475,7 @@ def add_parachute(
         sampling_rate=100,
         lag=0,
         noise=(0, 0, 0),
-        radius=1.5,
+        radius=None,
         height=None,
         porosity=0.0432,
     ):
@@ -1538,7 +1538,9 @@ def add_parachute(
             are in pascal.
         radius : float, optional
             Length of the non-unique semi-axis (radius) of the inflated hemispheroid
-            parachute. Default value is 1.5.
+            parachute. If not provided, it is estimated from ``cd_s`` assuming a
+            typical hemispherical parachute drag coefficient of 1.4, using the
+            formula: ``radius = sqrt(cd_s / (1.4 * pi))``.
             Units are in meters.
         height : float, optional
             Length of the unique semi-axis (height) of the inflated hemispheroid