Document engine plugins.

Rename `mjPLUGIN_DYNAMIC_LIBRARY_INIT` -> `mjPLUGIN_LIB_INIT`.

PiperOrigin-RevId: 521458377
Change-Id: I42aaec923e4f392899dfe53ab6ce1dddc75a83b3

doc/APIreference/APIfunctions.rst

@@ -43,6 +43,7 @@ API function can be classified as:
 Macros
 ^^^^^^
 
+
 .. _mjMARKSTACK:
 
 mjMARKSTACK
@@ -62,6 +63,7 @@ This macro is helpful when using the MuJoCo stack in custom computations. It wor
        // ... use temp as needed
        mjFREESTACK
 
+
 .. _mjFREESTACK:
 
 mjFREESTACK
@@ -73,6 +75,7 @@ mjFREESTACK
 
 Reset the MuJoCo stack pointer to the variable \_mark, normally saved by mjMARKSTACK.
 
+
 .. _mjDISABLED:
 
 mjDISABLED
@@ -85,6 +88,7 @@ mjDISABLED
 Check if a given standard feature has been disabled via the physics options, assuming mjModel\* m is defined. x is of
 type :ref:`mjtDisableBit`.
 
+
 .. _mjENABLED:
 
 mjENABLED
@@ -97,6 +101,7 @@ mjENABLED
 Check if a given optional feature has been enabled via the physics options, assuming mjModel\* m is defined. x is of
 type :ref:`mjtEnableBit`.
 
+
 .. _mjMAX:
 
 mjMAX
@@ -108,6 +113,7 @@ mjMAX
 
 Return maximum value. To avoid repeated evaluation with mjtNum types, use the function :ref:`mju_max`.
 
+
 .. _mjMIN:
 
 mjMIN
@@ -118,3 +124,23 @@ mjMIN
    #define mjMIN(a,b) (((a) < (b)) ? (a) : (b))
 
 Return minimum value. To avoid repeated evaluation with mjtNum types, use the function :ref:`mju_min`.
+
+
+.. _mjPLUGIN_LIB_INIT:
+
+mjPLUGIN_LIB_INIT
+~~~~~~~~~~~~~~~~~
+
+.. code-block:: C
+
+   #define mjPLUGIN_LIB_INIT                                                                 \
+     static void _mjplugin_dllmain(void);                                                    \
+     mjEXTERNC int __stdcall mjDLLMAIN(void* hinst, unsigned long reason, void* reserved) {  \
+       if (reason == 1) {                                                                    \
+         _mjplugin_dllmain();                                                                \
+       }                                                                                     \
+       return 1;                                                                             \
+     }                                                                                       \
+     static void _mjplugin_dllmain(void)
+
+Register a plugin as a dynamic library. See :ref:`plugin registration<exRegistration>` for more details.

doc/APIreference/APItypes.rst

@@ -26,6 +26,7 @@ MuJoCo defines a large number of types:
   - Structs for :ref:`abstract visualization<tyVisStructure>`.
   - Structs used by the :ref:`openGL renderer<tyRenderStructure>`.
   - Structs used by the :ref:`UI framework<tyUIStructure>`.
+  - Structs used by :ref:`engine plugins<tyPluginStructure>`.
 
 - Several :ref:`tyFunction` for user-defined callbacks.
 
@@ -555,6 +556,25 @@ Item types used in the UI framework.
 .. mujoco-include:: mjtItem
 
 
+.. _tyPluginEnums:
+
+Plugins
+^^^^^^^
+
+The enums below are defined in `mjplugin.h <https://github.com/deepmind/mujoco/blob/main/include/mujoco/mjplugin.h>`_.
+See :ref:`exPlugin` for details.
+
+
+.. _mjtPluginCapabilityBit:
+
+mjtPluginCapabilityBit
+~~~~~~~~~~~~~~~~~~~~~~
+
+Capabilities declared by an engine plugin.
+
+.. mujoco-include:: mjtPluginCapabilityBit
+
+
 
 .. _tyStructure:
 
@@ -838,7 +858,8 @@ This structure contains the custom OpenGL rendering context, with the ids of all
 User Interface
 ^^^^^^^^^^^^^^
 
-The names of these struct types are prefixed with ``mjui``.
+The names of these struct types are prefixed with ``mjui``, except for the main :ref:`mjUI` struct itself.
+
 
 .. _mjuiState:
 
@@ -890,6 +911,16 @@ This structure defines one section of the UI.
 .. mujoco-include:: mjuiSection
 
 
+.. _mjuiDef:
+
+mjuiDef
+~~~~~~~
+
+This structure defines one entry in the definition table used for simplified UI construction.
+
+.. mujoco-include:: mjuiDef
+
+
 .. _mjUI:
 
 mjUI
@@ -900,14 +931,23 @@ This structure defines the entire UI.
 .. mujoco-include:: mjUI
 
 
-.. _mjuiDef:
+.. _tyPluginStructure:
 
-mjuiDef
-~~~~~~~
+Plugins
+^^^^^^^
 
-This structure defines one entry in the definition table used for simplified UI construction.
+The names of these struct types are prefixed with ``mjp``. See :ref:`exPlugin` for more details.
 
-.. mujoco-include:: mjuiDef
+
+.. _mjpPlugin:
+
+mjpPlugin
+~~~~~~~~~
+
+This structure contains the definition of a single engine plugin. It mostly contains a set of callbacks, which are
+triggered by the compiler and the engine during various phases of the computation pipeline.
+
+.. mujoco-include:: mjpPlugin