Run C/C++ API docs workflow on PRs; publish only from main
- Add pull_request trigger with same paths filter as push (plus docs/c_cxx/**) so doc generation errors are caught earlier.
- Add docs/c_cxx/** to push paths as well so Doxyfile/header changes also trigger the workflow.
- Scope concurrency by ref and cancel in-progress non-main runs.
- Gate the artifact upload on refs/heads/main so PRs and non-main workflow_dispatch runs skip publishing.
- Fix stale comment: scheduled run is weekly, not monthly.
