Document RUST_LOG=trace for additional logging verbosity

[olivierlefloch]

Summary

This improves Contributing documentation to specifically mention trace level logging can be obtained via RUST_LOG=trace uv … as mentioned here: #1569 (comment)

Test Plan

Compare the output of

uv pip install --verbose requests

and

RUST_LOG=trace uv pip install --verbose requests

[zanieb]

Makes sense, probably better to have #1569 but until then... :)

Thank you!