Docs: document macOS CoreML EP builds and packages (#29923)
### Summary
- Documents how to build ONNX Runtime with the CoreML EP on **macOS**
(`--use_coreml`), with examples for shared library and Python wheel
builds.
- Clarifies that **official macOS release packages** (Python wheels via
`py-macos.yml`, and macOS C API / Java / Node.js packaging via
`mac-cpu-packing-jobs.yml`) are built with `--use_coreml`, so they
include the CoreML EP.
- Notes that custom/local builds omit CoreML unless `--use_coreml` is
passed.
- Cross-links from `docs/build/ios.md` and `docs/build/eps.md` so macOS
is no longer implied to be iOS-only.
### Validation
- Verified `--use_coreml` / `onnxruntime_USE_COREML` in
`tools/ci_build/build_args.py`, `tools/ci_build/build.py`, and
`cmake/CMakeLists.txt`.
- Verified official packaging enables CoreML:
- `tools/ci_build/github/azure-pipelines/templates/py-macos.yml`
(`--use_coreml`)
-
`tools/ci_build/github/azure-pipelines/templates/mac-cpu-packing-jobs.yml`
(`--use_coreml`)
- Docs-only change; matched nearby EP doc style (Install / Build
subsections).
Fixes #26689
### AI/LLM disclosure
- AI coding tools (including Grok and/or Codex agent-assisted editing)
were used to help draft or modify documentation and this PR description.
- I reviewed the complete change, understand the reasoning, and verified
the documented flags/packaging claims against the repository sources
listed above before submitting.
- This submission is original work of authorship under the project CLA /
contributor terms; AI output was not pasted unreviewed.
