docs: add NVLink pair diagnostic guidance

[pengfaming]

Summary

• add pairwise NVLink diagnostic guidance to the README
• show how to identify topology and GPU UUIDs before isolating a pair
• clarify directional and bidirectional bandwidth interpretation

Validation

• git diff --check
• checked Markdown code fences and referenced CLI options against the current README

Non-duplication and disclosure

• checked current open nvbandwidth PRs; this README-only NVLink diagnostic guidance does not overlap with feat: support multinode for json format output #54's multinode JSON output changes
• AI assistance was used to prepare this documentation change; I reviewed every changed line