Update MuJoCo version to 3.9.0 following the 3.8.1 release

PiperOrigin-RevId: 913596257
Change-Id: Ib8b27fd2224e38560a209054c1d508a9bba21bbf

doc/APIreference/APIglobals.rst

@@ -388,7 +388,7 @@ Defined in `mujoco.h <https://github.com/google-deepmind/mujoco/blob/main/includ
      - value
      - description
    * - ``mjVERSION_HEADER``
-     - 3008001
+     - 3009000
      - The version of the MuJoCo headers. This is an integer calculated from the version string "S.M.P"
        using the formula ``(S * 1e6) + (M * 1e3) + P``. For example, version 4.2.1 is represented as 4002001.
        The API function :ref:`mj_version` returns a number with the same meaning

doc/unity.rst

@@ -37,14 +37,14 @@ _____
 
 The MuJoCo app needs to be run at least once before the native library can be used, in order to register the library as
 a trusted binary. Then, copy the dynamic library file from
-``/Applications/MuJoCo.app/Contents/Frameworks/mujoco.framework/Versions/Current/libmujoco.3.8.1.dylib`` (it can be
+``/Applications/MuJoCo.app/Contents/Frameworks/mujoco.framework/Versions/Current/libmujoco.3.9.0.dylib`` (it can be
 found by browsing the contents of ``MuJoCo.app``) and rename it as ``mujoco.dylib``.
 
 Linux
 _____
 
 Expand the ``tar.gz`` archive to ``~/.mujoco``. Then copy the dynamic library from
-``~/.mujoco/mujoco-3.8.1/lib/libmujoco.so.3.8.1`` and rename it as ``libmujoco.so``.
+``~/.mujoco/mujoco-3.9.0/lib/libmujoco.so.3.9.0`` and rename it as ``libmujoco.so``.
 
 Windows
 _______