Document clang-tidy flow and drop legacy CI job

[RAMitchell]

Summary

Update the contributor documentation to use the clang-CUDA compile-database clang-tidy flow and remove the legacy CI job that still ran the old wrapper script.

What changed

• document ops/pipeline/run-clang-tidy-clang-cuda.sh as the local clang-tidy entry point
• show how to scope runs with XGBOOST_TIDY_FILES, XGBOOST_TIDY_CHECKS, and helper arguments
• remove the old CI job and keep the compile-database path as the canonical clang-tidy job

Testing

• pre-commit run --files .github/workflows/lint.yml doc/contrib/coding_guide.rst

[Copilot]

Pull request overview

Updates XGBoost contributor documentation and CI to standardize on the clang-generated CUDA compile database flow for clang-tidy, removing the legacy CI job that ran the older wrapper.

Changes:

• Document ops/pipeline/run-clang-tidy-clang-cuda.sh as the recommended local clang-tidy entry point, including scoping via env vars and CLI flags.
• Remove the legacy clang-tidy CI job and keep the compile-database-based job as the canonical workflow path.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
doc/contrib/coding_guide.rst	Replaces local clang-tidy instructions with the clang-CUDA compile DB helper and adds usage examples/configuration knobs.
.github/workflows/lint.yml	Drops the legacy clang-tidy CI job and retains the compile DB clang-tidy job under the clang-tidy job key.

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