Update benchmark code location docs

[Haihan-Jiang]

What

Updates the benchmark documentation to point at the current benchmarks/ directory on master instead of the old standalone benchmark branch. The old branch and the referenced benchmarks/cpp/README.txt file are no longer present, so this also notes that the comparison table is historical and that not every third-party comparison harness is in the current tree.

Fixes #8893.

Testing

• git diff --check -- docs/source/benchmarks.md
• mkdocs build -f docs/mkdocs.yml --strict in a temporary venv

[Haihan-Jiang]

This is ready for review. The benchmark code-location docs are updated, and the visible checks are green.