Add new mju_threadpool API function, and delete old threading API.

PiperOrigin-RevId: 922838541
Change-Id: Id9f7e0fb298ffde61fcc49a802dc78971858ce51

doc/programming/index.rst

@@ -31,9 +31,8 @@ OpenGL renderer
    state-of-the-art rendering engines (and can be replaced with such an engine if desired) but nevertheless it provides
    efficient and informative 3D rendering.
 Thread
-   The threading framework is written in C++ and exposed in C. It provides a :ref:`mjThreadPool<mjThreadPool>` interface
-   to process tasks asynchronously. To enable use in MuJoCo, create a thread pool and assign it to the
-   ``mjData.threadpool`` field.
+   The threading framework is written in C++ and exposed in C. It provides a thread pool interface
+   to process tasks asynchronously. To enable use in MuJoCo, call ``mju_threadpool``.
 UI framework
    The UI framework is written in C. UI elements are rendered in OpenGL. It has its own event
    mechanism and abstract hooks for keyboard and mouse input. The code samples use it with GLFW, but it can also be used
@@ -171,8 +170,6 @@ links below, to make this documentation self-contained.
    Defines enums and structs used for :doc:`procedural model editing <modeledit>`.
 `mjplugin.h <https://github.com/google-deepmind/mujoco/blob/main/include/mujoco/mjplugin.h>`__
    Defines data structures required by :ref:`engine plugins<exPlugin>`.
-`mjthread.h <https://github.com/google-deepmind/mujoco/blob/main/include/mujoco/mjthread.h>`__
-   Defines data structures and functions required by :ref:`thread<Thread>`.
 `mjmacro.h <https://github.com/google-deepmind/mujoco/blob/main/include/mujoco/mjmacro.h>`__
    Defines C macros that are useful in user code.
 `mjxmacro.h <https://github.com/google-deepmind/mujoco/blob/main/include/mujoco/mjxmacro.h>`__

doc/programming/samples.rst

@@ -60,7 +60,7 @@ Where the command line arguments are
   keyframe named "test" is present in the model, it is used as the initial state.
 - The ``ctrlnoise`` argument prevents models from settling into a static state where, due to warmstarts, one can
   measure artificially faster simulation.
-- When ``npoolthread > 1`` is specified, an engine-internal :ref:`mjThreadPool` is created with the specified number of
+- When ``npoolthread > 1`` is specified, an engine-internal thread pool is created with the specified number of
   threads, to speed up simulation of large scenes. Note that while it is possible to use both ``nthread`` and
   ``npoolthread``, the scenarios for which one would want these different types of multithreading are usually mutually
   exclusive.

doc/programming/simulation.rst

@@ -562,10 +562,9 @@ external force computed by inverse dynamics.
 Multi-threading
 ~~~~~~~~~~~~~~~
 
-MuJoCo has experimental support for within-step multi-threading. When a :ref:`mjThreadPool` is assigned to
-``mjData.threadpool``, parts of the simulation pipeline — such as collision detection and constraint solving across
-:ref:`islands<siSleep>` — can be distributed across worker threads. Note that within-step threading currently has
-significant memory overhead and is still a work in progress.
+MuJoCo has support for within-step multi-threading. When a thread pool is initialized via
+``mju_threadpool``, parts of the simulation pipeline — such as collision detection and constraint solving across
+:ref:`islands<siSleep>` — can be distributed across worker threads.
 
 The more common and well-supported use of multi-threading is to speed up sampling operations that are
 common in more advanced applications. Simulation is inherently serial over time (the output of one mj_step is the