Add implicit stiffness for flex_interp to mj_implicitSkip.

PiperOrigin-RevId: 867706885
Change-Id: Ic94c65b618a415609bffe3d69a86f9034f2d2400

doc/XMLreference.rst

@@ -3575,7 +3575,7 @@ saving the XML:
 
 .. _body-flexcomp-dof:
 
-:at:`dof`: :at-val:`[full, radial, trilinear], "full"`
+:at:`dof`: :at-val:`[full, radial, trilinear, quadratic], "full"`
    The parametrization of the flex's degrees of freedom (dofs). See the video on the right illustrating the
    different parametrizations with deformable spheres. The three models in the video are respectively
    `sphere_full <https://github.com/google-deepmind/mujoco/blob/main/model/flex/sphere_full.xml>`__,

doc/changelog.rst

@@ -20,6 +20,10 @@ Significant new features
 - Added new :ref:`flexvert<equality-flexvert>` equality constraints that enable cloth simulations with coarser meshes.
   This adds a new option ``vert`` to flexcomp edge :ref:`equality<flexcomp-edge-equality>` and the new equality type
   :ref:`flexvert<equality-flexvert>`.
+- Added implicit integration support for deformable objects (flex) in ``implicit`` and ``implicitfast``
+  :ref:`integrators<geIntegration>`. This method extracts the flex degrees of freedom and solves them as a dense block,
+  enabling increased stability for stiff flex objects without reducing the timestep. It is compatible with the
+  ``trilinear`` and ``quadratic`` :ref:`dof<body-flexcomp-dof>` types.
 
 .. container:: custom-clear