Document RUST_LOG=trace for additional logging verbosity (#1670)

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## 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
```

CONTRIBUTING.md

@@ -80,3 +80,11 @@ RUST_LOG=uv=info TRACING_DURATIONS_FILE=target/traces/jupyter.ndjson cargo run -
 ```shell
 RUST_LOG=uv=info TRACING_DURATIONS_FILE=target/traces/jupyter.ndjson cargo run --features tracing-durations-export --bin uv-dev --profile profiling -- resolve jupyter
 ```
+
+### Trace-level logging
+
+You can enable `trace` level logging using the `RUST_LOG` environment variable, i.e.
+
+```shell
+RUST_LOG=trace uv …
+```