Change simulate plugin directory name to mujoco_plugin.

This helps distro packagers avoid having to create a directory with the generic `plugin` name in system locations.

Context: https://github.com/deepmind/mujoco/issues/552
PiperOrigin-RevId: 489011386
Change-Id: Ie9f1748cbb4f2b9dc8c328064caaeb50c88fdec3

doc/changelog.rst

@@ -51,6 +51,12 @@ General
   - 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>`_.
 
+Simulate
+^^^^^^^^
+
+- Renamed the directory in which the ``simulate`` application searches for plugins from ``plugin`` to ``mujoco_plugin``.
+
+
 Version 2.3.0 (October 18, 2022)
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