[libc-commits] [libc] [libc] Add the `<endian.h>` header. (PR #125168)

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c8ef wrote:

> While you're here, do you mind wiring up docgen so we generate documentation for this newly added header? [d398c0c](https://github.com/llvm/llvm-project/commit/d398c0c97aa0bfaeed5647f75bc37c87b8142f79) is a good example.

Thank you for your guidance! Done.

https://github.com/llvm/llvm-project/pull/125168