fix(doc): Correct unexpected "testcases 说明" page in Doxygen output (#10888)

Problem description:
The `src/utest/perf/README.md` file was incorrectly displayed in the root level of the Doxygen navigation tree due to missing `@subpage` reference. This caused confusion and made the documentation harder to navigate.

Problem analysis:
Since the `README.md` file wasn't associated with a parent page via the `@subpage` directive, Doxygen placed it at the root level of the navigation tree instead of nesting it correctly under the "utest Framework" section.

Solution:
The issue is resolved by:
1. Assigning a unique page ID (`page_component_utest_perf_cases`) and an English title ("utest Performance Test Cases") to the `README.md` file.
2. Updating the `README.md` content to be bilingual, for better alignment with the English documentation.
3. Modifying `documentation/utest.md` to properly reference this page via a `@subpage` directive, ensuring it is correctly nested under the "utest Framework" section.

This issue was originally introduced by the following commit:
Commit: 7499790
Title: utest: core: move perf testcase from example to src

Signed-off-by: lhxj <[email protected]>

Author: lhxj

documentation/6.components/utest/utest.md

@@ -201,6 +201,8 @@ A basic test case must contain the following:
 - test_xxx(void)
 
     Test implementation of each functional unit. The user determines the function name and function implementation based on the requirements.
+    
+    As a concrete example, the utest module provides a set of performance test cases (located in the src/utest/perf/ directory), which are implemented as a series of test_xxx(void) functions. For more details, see @subpage page_component_utest_perf_cases.
 
 - uassert_true
 

src/utest/perf/README.md

@@ -1,12 +1,12 @@
-# testcases 说明
+@page page_component_utest_perf_cases utest Performance Test Cases
 
-## 一、测试用例说明
+# 测试用例说明 (Test Cases Description)
 
-| 文件  | 说明 |
-|--------|--------|
-| context_switch.c  | 上下文切换测试代码  |
-| irq_latency.c  | 中断延时测试代码  |
-| rt_perf_thread_event.c  | 线程事件性能测试  |
-| rt_perf_thread_mbox.c  | 线程邮箱性能测试  |
-| rt_perf_thread_mq.c  | 线程消息队列性能测试  |
-| rt_perf_thread_sem.c  | 线程信号量性能测试  |
+| 文件 (File) | 说明 (Description) |
+|--------------|--------------------|
+| context_switch.c | 上下文切换测试代码 (Context switch performance test) |
+| irq_latency.c | 中断延时测试代码 (Interrupt latency test) |
+| rt_perf_thread_event.c | 线程事件性能测试 (Thread event performance test) |
+| rt_perf_thread_mbox.c | 线程邮箱性能测试 (Thread mailbox performance test) |
+| rt_perf_thread_mq.c | 线程消息队列性能测试 (Thread message queue performance test) |
+| rt_perf_thread_sem.c | 线程信号量性能测试 (Thread semaphore performance test) |