nxcodec: add .rst documentation

Signed-off-by: shizhenghui <shizhenghui@xiaomi.com>

Documentation/applications/system/nxcodec/index.rst

@@ -0,0 +1,45 @@
+================================================
+``nxcodec`` NxCodec video codec test application
+================================================
+
+This application is a command-line tool that verifies the functionality of
+NuttX codecs. Specifically, it serves as a verification tool for v4l2m2m.
+
+Usage
+-----
+
+This page shows nxcodec options, For a complete list of nxcodec options
+just run ``nxcodec -h``::
+
+ ap> nxcodec -h
+ NxCodec Version: 1.00
+ Usage: nxcodec -d devname -s [wxh] -f [informt] -i infile -f [outformat] -o outfile
+ Default settings for decoder parameters
+
+ [-d | --device]  Video device name
+ [-s | --size]    Size of stream
+ [-h | --help]    Print this message
+ [-f | --format]  Format of stream
+ [-i | --infile]  Input filename for M2M devices
+ [-o | --outfile] Outputs stream to filename
+
+By default, it is the decodeing mode, with a default parameter size of
+640x480, input format of H264, and output format of YUV420. The ``-i``
+parameter corresponds to the input parameter before it, and the ``-i``
+parameter corresponds to the output parameter after it.
+
+Examples
+--------
+
+Decode an H264 stream file into a yuv420 file::
+
+ mount -t hostfs -o fs=/path/from/ /stream
+ nxcodec -d /dev/video1 -s 256x144 -i /stream/256x144.h264 -o /stream/256x144-yuv420p.yuv
+
+Encode a yuv420 file as an h264 stream file::
+
+ mount -t hostfs -o fs=/path/from/ /stream
+ nxcodec -d /dev/video2 -s 256x144 -f YU12 -i /stream/256x144-yuv420p.yuv -f H264 -o /stream/256x144.h264
+
+Author: ZhengHui SHI
+Date: 9 July 2024