[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.
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Co-authored-by: siya <siya@Siya.com>
d398c0c97a