fix(doc): Exclude 'src/utest/perf/README.md' from Doxygen build (RT-Thread#10888)

Problem description:
The file 'src/utest/perf/README.md' was incorrectly appearing as a root-level page in the Doxygen documentation output.

Problem analysis:
This file was unintentionally included by the Doxygen build process following commit 7499790. It is an internal README for performance tests and is not intended to be part of the generated documentation.

Solution:
The issue is resolved by explicitly excluding this file from the build. The path '../src/utest/perf/README.md' has been added to the EXCLUDE list in both 'documentation/Doxyfile.1.9.1' and 'documentation/Doxyfile.1.9.8'.

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 <2743257167@qq.com>

documentation/Doxyfile.1.9.1

@@ -924,7 +924,8 @@ RECURSIVE              = YES
 EXCLUDE                = ./0.doxygen/mainpage.h \
                          ./2.quick-start/quick_start_qemu \
                          ./env/env-vscode.md \
-                         ./7.contribution/coding_style_cn.md
+                         ./7.contribution/coding_style_cn.md \
+                         ../src/utest/perf/README.md
 
 # The EXCLUDE_SYMLINKS tag can be used to select whether or not files or
 # directories that are symbolic links (a Unix file system feature) are excluded

documentation/Doxyfile.1.9.8

@@ -1014,7 +1014,8 @@ RECURSIVE              = YES
 EXCLUDE                = ./0.doxygen/mainpage.h \
                          ./2.quick-start/quick_start_qemu \
                          ./env/env-vscode.md \
-                         ./7.contribution/coding_style_cn.md
+                         ./7.contribution/coding_style_cn.md \
+                         ../src/utest/perf/README.md
 
 # The EXCLUDE_SYMLINKS tag can be used to select whether or not files or
 # directories that are symbolic links (a Unix file system feature) are excluded