[libc][cpio] Add cpio.h header. (#123798)

[libc][docs] add cpio to documentation and include related functi… 
These changes ensure that the cpio header is documented properly
with respect to the issue
(https://github.com/llvm/llvm-project/issues/122006 ).

**Changes:**
1. **cpio.yaml**: Created a new YAML file for cpio with functions
and related macros.
2. **CMakeLists.txt**: Added cpio to the documentation
directories.
3. **index.rst**: Included `cpio` in the documentation index.

---------

Co-authored-by: siya <siya@Siya.com>

libc/docs/CMakeLists.txt

@@ -37,6 +37,7 @@ if (SPHINX_FOUND)
       aio
       arpa/inet
       assert
+      cpio
       ctype
       errno
       fenv

libc/docs/headers/index.rst

@@ -8,6 +8,7 @@ Implementation Status
    arpa/inet
    assert
    complex
+   cpio
    ctype
    errno
    fenv

libc/utils/docgen/cpio.yaml

@@ -0,0 +1,44 @@
+macros:
+  C_IRUSR:
+    in-latest-posix: ''
+  C_IWUSR:
+    in-latest-posix: ''
+  C_IXUSR:
+    in-latest-posix: ''
+  C_IRGRP:
+    in-latest-posix: ''
+  C_IWGRP:
+    in-latest-posix: ''
+  C_IXGRP:
+    in-latest-posix: ''
+  C_IROTH:
+    in-latest-posix: ''
+  C_IWOTH:
+    in-latest-posix: ''
+  C_IXOTH:
+    in-latest-posix: ''
+  C_ISUID:
+    in-latest-posix: ''
+  C_ISGID:
+    in-latest-posix: ''
+  C_ISVTX:
+    in-latest-posix: ''
+  C_ISDIR:
+    in-latest-posix: ''
+  C_ISFIFO:
+    in-latest-posix: ''
+  C_ISREG:
+    in-latest-posix: ''
+  C_ISBLK:
+    in-latest-posix: ''
+  C_ISCHR:
+    in-latest-posix: ''
+  C_ISCTG:
+    in-latest-posix: ''
+  C_ISLNK:
+    in-latest-posix: ''
+  C_ISSOCK:
+    in-latest-posix: ''
+  MAGIC:
+    in-latest-posix: ''
+