Fix C API docs Doxygen download (#32210)
## Description
Replace the obsolete `doxygen.nl` download URL for the pinned Doxygen
1.9.8 Linux binary with the official asset from the Doxygen GitHub
release tag `Release_1_9_8`.
The C API docs workflow has persistently failed while downloading
Doxygen (runs
[31917039689](https://github.com/microsoft/onnxruntime/actions/runs/31917039689),
[31975339915](https://github.com/microsoft/onnxruntime/actions/runs/31975339915),
and
[32050624667](https://github.com/microsoft/onnxruntime/actions/runs/32050624667)).
Because no fresh `onnxruntime-c-apidocs` artifact was produced before
the previous artifact expired, the downstream ONNX Publish site run
[32533178465](https://github.com/microsoft/onnxruntime/actions/runs/32533178465)
failed at `Download C apidocs artifact`, blocking GitHub Pages
deployment.
This is intentionally separate from accessibility PR #32180.
## Validation
- Confirmed the official release URL follows one redirect and returns
HTTP 200.
- Confirmed GitHub release tag `Release_1_9_8` contains
`doxygen-1.9.8.linux.bin.tar.gz` (50,500,806 bytes).
- Downloaded the archive successfully and verified gzip/tar integrity.
- Confirmed extraction produces `doxygen-1.9.8/bin/doxygen` while
preserving the workflow's existing extraction and invocation paths.
- Parsed the workflow as valid YAML.
- Dispatched the [C/C++ API docs workflow from this
branch](https://github.com/microsoft/onnxruntime/actions/runs/32534100983);
the install, Doxygen generation, and site staging steps succeeded, and
the log reports `Doxygen version used: 1.9.8`. Artifact upload was
skipped as expected because the ref is not `main`.
## Recovery after merge
1. Dispatch the **Update C/C++ API Docs** workflow on `main`.
2. Verify it uploads a fresh `onnxruntime-c-apidocs` artifact.
3. Rerun the failed GitHub Pages jobs from ONNX Publish site run
32533178465.
No Doxygen version or unrelated workflow behavior is changed.
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
