Documentation/applications/system/critmon: add critmon man page

Add documentation for the critmon command including:
- critmon (one-shot dump)
- critmon_start (start daemon)
- critmon_stop (stop daemon)
- Output format explanation
- Configuration options

Signed-off-by: hanzhijian <hanzhijian@zepp.com>

Documentation/applications/system/critmon/index.rst

@@ -0,0 +1,91 @@
+====================================
+``critmon`` critical section monitor
+====================================
+
+The ``critmon`` command displays critical section and pre-emption timing
+information for all tasks and threads in the system. It reads from the
+procfs ``critmon`` and ``status`` files for each process/thread.
+
+Three commands are provided:
+
+- ``critmon``: One-shot dump of critical section timing data.
+- ``critmon_start``: Start the background monitoring daemon.
+- ``critmon_stop``: Stop the background monitoring daemon.
+
+The daemon periodically samples critical section usage at a configurable
+interval (default: 2 seconds) and prints the timing information.
+
+Configuration
+=============
+
+- ``CONFIG_SCHED_CRITMONITOR``: Enable the critical section monitor
+  in the kernel (required dependency).
+- ``CONFIG_FS_PROCFS``: Enable procfs filesystem support.
+- ``CONFIG_SYSTEM_CRITMONITOR``: Enable the critmon system tool.
+
+The following additional options are available:
+
+- ``CONFIG_SYSTEM_CRITMONITOR_DAEMON_STACKSIZE`` - Stack size for the
+  daemon task (default: 2048).
+- ``CONFIG_SYSTEM_CRITMONITOR_DAEMON_PRIORITY`` - Priority for the
+  daemon task (default: 50).
+- ``CONFIG_SYSTEM_CRITMONITOR_INTERVAL`` - Sampling interval in
+  seconds (default: 2).
+- ``CONFIG_SYSTEM_CRITMONITOR_MOUNTPOINT`` - procfs mount point
+  (default: ``/proc``).
+
+Usage
+=====
+
+.. code-block:: text
+
+   critmon
+   critmon_start
+   critmon_stop
+
+Output Format
+=============
+
+The output shows a table with the following columns:
+
+- **PRE-EMPTION CALLER**: The function that most recently enabled
+  pre-emption for this task.
+- **CSECTION CALLER**: The function that most recently entered a
+  critical section.
+- **RUN**: Maximum time spent in a critical section.
+- **TIME**: Total accumulated critical section time.
+- **PID**: Process/thread ID.
+- **DESCRIPTION**: Task name (if ``CONFIG_TASK_NAME_SIZE > 0``).
+
+Examples
+========
+
+One-shot dump of critical section timing:
+
+.. code-block:: text
+
+   nsh> critmon
+   PRE-EMPTION CALLER            CSECTION CALLER               RUN              TIME             PID   DESCRIPTION
+   nxtask_exit                   nxtask_init                   0.000010000      0.000010000      0     Idle_Task
+   nxtask_init                   up_irq_save                   0.000005000      0.000015000      1     hpwork 0x10001000
+
+Start the monitoring daemon (prints every 2 seconds):
+
+.. code-block:: text
+
+   nsh> critmon_start
+   Csection Monitor: Started: 5
+
+Stop the monitoring daemon:
+
+.. code-block:: text
+
+   nsh> critmon_stop
+   Csection Monitor: Stopping: 5
+   Csection Monitor: Stopped: 5
+
+See Also
+========
+
+- :doc:`../stackmonitor/index`
+- :doc:`../gprof/index`