docs: document quicklink.modern.mjs bundle

[sankalpsthakur]

Summary

• Documents the quicklink.modern.mjs build in README.md and the site usage section.
• Notes when to prefer it (modern browsers / native ES modules; smaller, no legacy transforms) and includes an import example.

Fixes #385

Validation

• Confirmed dist/quicklink.modern.mjs is produced by the microbundle build and tracked in .size-limit.json.
• Docs-only change; no code or build changes.

AI/LLM disclosure

• AI coding tools (including Grok and/or Codex agent-assisted editing) were used to help draft or modify code and this PR description.
• I reviewed the complete change, understand the reasoning, and ran the reported local tests before submitting.
• This submission is original work of authorship under the project CLA / contributor terms; AI output was not pasted unreviewed.

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