docs: add gprof usage documentation

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

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Documentation/applications/system/gprof/index.rst

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+=============================
+``gprof`` GNU Profile tool
+=============================
+
+GNU Profile (gprof) is a performance analysis tool that helps developers
+identify code bottlenecks and optimize their programs.
+It provides detailed information about the execution time and call
+frequency of functions within a program.
+
+gprof can be used to:
+
+1. Detect performance bottlenecks in your code
+2. Identify which functions consume the most execution time
+3. Analyze the call graph of your program
+4. Help prioritize optimization efforts
+
+Usage
+=====
+
+Build
+-----
+
+Enable the following configuration in NuttX::
+
+  CONFIG_SYSTEM_GPROF
+
+Using in NuttX
+--------------
+
+1. Start profiling::
+
+     nsh> gprof start
+
+2. Stop profiling::
+
+     nsh> gprof stop
+
+3. Dump profiling data::
+
+     nsh> gprof dump /tmp/gmon.out
+
+Analyzing on Host
+-----------------
+
+1. Pull the profiling data to host::
+
+     adb pull /tmp/gmon.out ./gmon.out
+
+2. Analyze the data using gprof tool::
+
+     The saved file format complies with the standard gprof format.
+     For detailed instructions on gprof command usage, please refer to the GNU gprof manual:
+     https://ftp.gnu.org/old-gnu/Manuals/gprof-2.9.1/html_mono/gprof.html
+
+     arm-none-eabi-gprof ./nuttx/nuttx gmon.out -b
+
+     Example output:
+
+     ```
+     arm-none-eabi-gprof nuttx/nuttx gmon.out -b
+     Flat profile:
+
+     Each sample counts as 0.001 seconds.
+       %   cumulative   self              self     total
+      time   seconds   seconds    calls   s/call   s/call  name
+      66.41      3.55     3.55       43     0.08     0.08  sdelay
+      33.44      5.34     1.79       44     0.04     0.04  delay
+       0.07      5.34     0.00                             up_idle
+       0.04      5.34     0.00                             nx_start
+       0.02      5.34     0.00                             fdtdump_main
+       0.02      5.34     0.00                             nxsem_wait
+       0.00      5.34     0.00        1     0.00     5.34  hello_main
+       0.00      5.34     0.00        1     0.00     0.00  singal_handler
+
+     ```
+
+     This output shows the performance profile of the program,
+     including execution time and call counts for each function.
+     The flat profile table provides a quick overview of where the program spends most of its time.
+     In this example, `sdelay` and `delay` functions consume the majority of execution time.
+     This information can be used to identify performance bottlenecks and optimize critical parts of the code.