docs: add benchmarking guidelines for cp

[mattsu2020]

Document recommended datasets, hyperfine usage, and considerations for measuring
large sequential copies, metadata-heavy workloads, reflink behaviour, and
sparse files.

[sylvestre]

nope, i would like to see actual benchmarks.
see:
src/uu/*/benches

[mattsu2020]

add cp_bench

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)

[codspeed-hq]

CodSpeed Performance Report

Merging #8807 will not alter performance

_{Comparing mattsu2020:cp_benchmarks (773348f) with main (be25344)}

Summary

✅ 69 untouched
🆕 6 new
⏩ 73 skipped¹

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
🆕	cp_archive_balanced_tree[(5, 4, 10)]	N/A	79.7 ms	N/A
🆕	cp_large_file[16]	N/A	294.1 µs	N/A
🆕	cp_preserve_metadata[(5, 4, 10)]	N/A	77 ms	N/A
🆕	cp_recursive_balanced_tree[(5, 4, 10)]	N/A	74.4 ms	N/A
🆕	cp_recursive_deep_tree[(120, 4)]	N/A	13.7 ms	N/A
🆕	cp_recursive_wide_tree[(6000, 800)]	N/A	192.8 ms	N/A

Footnotes

1. 73 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩