Documentation: Add Whetstone FPU benchmark documentation

Add documentation for the Whetstone floating-point benchmark
application which is being added to nuttx-apps.

The documentation includes:
- Overview of the benchmark
- Configuration options
- Usage examples
- Output interpretation
- Test modules description

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

Documentation/applications/benchmarks/whetstone/index.rst

@@ -0,0 +1,170 @@
+=========================================
+``whetstone`` Whetstone FPU Benchmark
+=========================================
+
+Overview
+========
+
+Whetstone is a classic synthetic benchmark program designed to evaluate
+processor arithmetic performance. By executing a series of standardized
+computational tasks, it provides accurate assessment of floating-point
+and integer operation capabilities.
+
+This implementation is a C converted version of the Double Precision
+Whetstone benchmark, based on the work by Rich Painter (Painter Engineering,
+Inc.) from the netlib.org version.
+
+The test results help developers:
+
+- Quantify FPU (Floating-Point Unit) performance
+- Analyze the impact of compiler optimization levels (-O2, -O3, etc.)
+  on code execution efficiency
+- Compare arithmetic performance across different hardware platforms
+  or system configurations
+
+Configuration
+=============
+
+Before running the test, enable the following Kconfig options:
+
+.. code-block:: bash
+
+   # Enable custom permissive license components
+   CONFIG_ALLOW_CUSTOM_PERMISSIVE_COMPONENTS=y
+
+   # Enable Whetstone benchmark
+   CONFIG_BENCHMARK_WHETSTONE=y
+
+   # Whetstone requires floating-point support in the C library
+   CONFIG_LIBC_FLOATINGPOINT=y
+
+Additional configuration options:
+
+- ``CONFIG_BENCHMARK_WHETSTONE_PROGNAME`` - Program name (default: "whetstone")
+- ``CONFIG_BENCHMARK_WHETSTONE_PRIORITY`` - Task priority (default: 100)
+- ``CONFIG_BENCHMARK_WHETSTONE_STACKSIZE`` - Stack size (default: DEFAULT_TASK_STACKSIZE)
+
+Usage
+=====
+
+Command Syntax
+--------------
+
+.. code-block:: bash
+
+   whetstone [-c] [loops]
+
+Parameters
+----------
+
+- ``[loops]`` - Module loop count. Sets the number of iterations for each
+  internal test module. Increasing this value significantly increases
+  computation and execution time. Default: 1000
+- ``-c`` - Continuous mode. When specified, the benchmark repeats indefinitely
+  until interrupted. Default: disabled
+
+Examples
+--------
+
+Run a standard test with default parameters (1000 loops):
+
+.. code-block:: bash
+
+   nsh> whetstone
+
+Increase computation load per module (100000 loops):
+
+.. code-block:: bash
+
+   nsh> whetstone 100000
+
+Run in continuous mode with custom loop count:
+
+.. code-block:: bash
+
+   nsh> whetstone -c 100000
+
+Output Interpretation
+=====================
+
+After completion, whetstone outputs test configuration, total duration,
+and the final performance score.
+
+Example Output
+--------------
+
+.. code-block:: bash
+
+   nsh> whetstone 100000
+
+   Loops: 100000, Iterations: 1, Duration: 5765 millisecond.
+   C Converted Double Precision Whetstones: 1.7 MIPS
+
+- **Loops: 100000** - Each module executed 100,000 loop iterations
+- **Iterations: 1** - The test suite ran 1 round
+- **Duration: 5765 millisecond** - Total execution time
+- **1.7 MIPS** - Final performance score
+
+Performance Metrics
+-------------------
+
+**MIPS / KIPS**
+
+- **Definition**: Whetstone performance units - MIPS (Mega Whetstone
+  Instructions Per Second) and KIPS (Kilo Whetstone Instructions Per Second)
+- **Calculation**: ``KIPS = (100.0 * loops * iterations) / (duration_sec * 1000)``
+- **Interpretation**: Higher scores indicate better processor arithmetic
+  performance
+- **Unit conversion**: Results display as KIPS when below 1000, otherwise
+  as MIPS
+
+Test Modules
+============
+
+The Whetstone benchmark consists of 11 carefully designed computational
+modules covering different operation types:
+
+.. list-table::
+   :header-rows: 1
+
+   * - Module
+     - Description
+   * - Module 1
+     - Simple identifiers - basic floating-point operations
+   * - Module 2
+     - Array elements - array-based floating-point operations
+   * - Module 3
+     - Array as parameter - procedure calls with array arguments
+   * - Module 4
+     - Conditional jumps - branch operations
+   * - Module 5
+     - (Omitted)
+   * - Module 6
+     - Integer arithmetic - complex integer operations
+   * - Module 7
+     - Trigonometric functions - sin, cos, atan calculations
+   * - Module 8
+     - Procedure calls - function call overhead
+   * - Module 9
+     - Array references - array indexing operations
+   * - Module 10
+     - Integer arithmetic - simple integer operations
+   * - Module 11
+     - Standard functions - chained math functions (sqrt, exp, log)
+
+Notes
+=====
+
+- This benchmark uses double-precision floating-point arithmetic
+- For accurate measurements, ensure the system is not under heavy load
+- If "Insufficient duration" is reported, increase the loop count
+- Timing precision is in milliseconds, enabling quick and accurate results
+  even with fewer loop iterations on high-performance embedded CPUs
+
+References
+==========
+
+- Original Whetstone benchmark: H.J. Curnow and B.A. Wichmann,
+  "A Synthetic Benchmark", The Computer Journal, Vol 19, No 1,
+  February 1976, pp. 43-49
+- netlib.org Whetstone: https://www.netlib.org/benchmark/whetstone.c