[libc][docs] Add sys/utsname header and documentation for uname function (#131817)

These changes tracks `utsname.h` for the implementation status of
functions and macros, with respect to the issue (
https://github.com/llvm/llvm-project/issues/122006 ).

libc/docs/CMakeLists.txt

@@ -67,6 +67,7 @@ if (SPHINX_FOUND)
       threads
       uchar
       unistd
+      sys/utsname
       wchar
       wctype
     )

libc/docs/headers/index.rst

@@ -40,6 +40,7 @@ Implementation Status
    time
    uchar
    unistd
+   sys/utsname
    wchar
    wctype
 ..

libc/utils/docgen/sys/utsname.yaml

@@ -0,0 +1,3 @@
+functions:
+  uname:
+    in-latest-posix: ''