Deformable solid plugin for particles.

Using coordinate-free formula equivalent to linear finite elements.

PiperOrigin-RevId: 491317615
Change-Id: I096b9a56075bbb630f64bfffac8a8d66457e5e2d

doc/changelog.rst

@@ -50,6 +50,19 @@ General
     <https://github.com/deepmind/mujoco/tree/main/test/benchmark/engine_core_smooth_benchmark_test.cc>`_.
   - 50% faster ``mju_dotSparse`` using manual loop unroll. See `engine_util_sparse_benchmark_test
     <https://github.com/deepmind/mujoco/tree/main/test/benchmark/engine_util_sparse_benchmark_test.cc>`_.
+- Added new :at:`solid` passive force plugin:
+
+  .. youtube:: AGcTGHbbze4
+     :align: right
+     :height: 150px
+
+  - This is new force field compatible with the :ref:`composite<body-composite>` particles.
+  - Generates a tetrahedral mesh having particles with mass concentrated at vertices.
+  - Uses a piecewise-constant strain model equivalent to finite elements but expressed in a coordinate-free
+    formulation. This implies that all quantities can be precomputed except edge elongation, as in a mass-spring model.
+  - Only suitable for small strains (large displacements but small deformations). Tetrahedra may invert if subject to
+    large loads.
+
 - Add API functions ``mj_loadPluginLibrary`` and  ``mj_loadAllPluginLibraries``. The first function is identical to
   ``dlopen`` on a POSIX system, and to ``LoadLibraryA`` on Windows. The second function scans a specified directory for
   all dynamic libraries file and loads each library found. Dynamic libraries opened by these functions are assumed to