Documentation: add nxcamera documentation

Document the nxcamera command usage, including device selection,
resolution, and pixel format options. Also mention recent support
for multi-instance camera mounting and the macOS AVFoundation
backend on the SIM platform.

Signed-off-by: Peter Bee <bijunda@bytedance.com>

Author: bijunda

Documentation/applications/system/nxcamera/index.rst

@@ -1,3 +1,111 @@
-============================================
-``nxcamera`` NxCamera video test application
-============================================
+=============================================
+``nxcamera`` Camera/Video Stream Test Command
+=============================================
+
+Introduction
+============
+
+``nxcamera`` is a command-line utility for testing camera devices and video
+stream capture in NuttX. It is built on top of the NuttX video subsystem
+(using a V4L2-style interface) and is typically used to:
+
+- Enumerate and open video device nodes such as ``/dev/video0`` and
+  ``/dev/video1``
+- Configure capture parameters including resolution and pixel format
+- Capture video frames for validation, debugging, or simple data dumping,
+  depending on platform support and build configuration
+
+Usage
+=====
+
+``nxcamera`` is an interactive command-line program. Start it from NSH, then
+enter commands at the ``nxcamera>`` prompt:
+
+.. code-block:: console
+
+   nsh> nxcamera
+   nxcamera>
+
+Tip: Execute NSH Commands (Hidden)
+==================================
+
+At the ``nxcamera>`` prompt you can run an NSH command by prefixing it with
+``!``. This is useful for quickly checking system state or invoking other
+utilities without exiting ``nxcamera``.
+
+.. code-block:: console
+
+   nxcamera> !ls /dev
+   /dev:
+    console
+    fb0
+    gpio0
+    gpio1
+    gpio2
+    gpio3
+    loop
+    null
+    oneshot
+    ram0
+    ram1
+    ram2
+    video0
+    video1
+    zero
+   nxcamera> !poweroff
+   bash>
+
+A typical workflow at the prompt is:
+
+- ``input /dev/video0`` to set the input video node.
+- ``output /dev/fb0`` to set the output node, for example a framebuffer.
+- ``stream 640 480 30 NV12`` to start streaming with ``width height fps
+  format``.
+- ``stop`` to stop streaming.
+
+You may just copy-paste the commands below to get started:
+
+.. code-block:: console
+
+  nxcamera
+  input /dev/video0
+  output /dev/fb0
+  stream 640 480 30 YUYV # or NV12 on macOS
+
+Pixel Format
+============
+
+For the ``stream`` command, the pixel format depends on the platform. On the
+macOS ``sim`` platform you may use ``NV12``, while on Linux systems ``YUYV``
+is more commonly used.
+
+Examples
+========
+
+1. Start ``nxcamera`` and configure a typical interactive capture session:
+
+.. code-block:: console
+
+   nsh> nxcamera
+   nxcamera> input /dev/video0
+   nxcamera> output /dev/fb0
+   nxcamera> stream 640 480 30 NV12
+   nxcamera> stop
+
+.. figure:: nxcamera_macos_sim.png
+   :alt: nxcamera running on the macOS SIM platform
+   :align: center
+
+   ``nxcamera`` using the macOS AVFoundation backend on the SIM platform.
+
+Features and Updates
+====================
+
+- Multiple camera instances are supported, allowing several cameras to be
+  exposed as different device nodes such as ``/dev/video0`` and
+  ``/dev/video1``. This makes it easier to select and validate different
+  video input sources on the same system.
+- On the ``sim`` platform, support has been added for the macOS
+  AVFoundation backend. This enables camera capture and functional
+  verification on macOS hosts, subject to build configuration and host
+  permission settings.