[#1376] Add release note snippet

Author: juan-g-bonilla

docs/source/Support/bskReleaseNotesSnippets/1375-refactor-mujoco-module-layout.rst

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+- ``MeanRevertingNoise`` abstract base class moved to ``mujocoDynamics/_GeneralModuleFiles``; importable as ``MJMeanRevertingNoise`` for Python subclassing
+- ``StochasticAtmDensity`` is now a standalone BSK module (``MJStochasticAtmDensity``) in its own folder
+- ``StochasticDragCoeff`` is now a standalone BSK module (``MJStochasticDragCoeff``) in its own folder
+- ``PIDControllers`` folder renamed to ``JointPIDController``; SWIG module renamed to ``MJJointPIDController``
+- Added ``.rst`` documentation pages for ``NBodyGravity``, ``JointPIDController``, ``StochasticAtmDensity``, and ``StochasticDragCoeff``