MAINTAINERS: add myself as owner of documentation (#10272)

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

MAINTAINERS

@@ -45,6 +45,10 @@
 
 # Below are existing maintainer entries, divided by module:
 
+tag: documentation
+path: documentation
+owners: Chen Wang(unicornx)<unicorn_wang@outlook.com>
+
 tag: libc
 path: components/libc
 owners: Meco Jianting Man(mysterywolf)<920369182@qq.com>