docs(examples): expand env template

[shizhigu]

Summary

• Expand examples/.env.example beyond the three LiveKit credentials.
• Add commented placeholders for provider, avatar, tracing, SIP, and primitive-example variables used by examples.
• Clarify that LiveKit Inference examples only need LiveKit Cloud credentials, while direct provider plugins need provider keys.

Tests

• uv run python - <<'PY' ... dotenv_values('examples/.env.example') ... PY
• make check
• git diff --check
• Claude reviewed the docs-only diff and found no incorrect env var names.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[shizhigu]

Closing this standalone env-template PR because the change is now folded into #5593. Keeping the examples first-run docs fixes together should be easier to review than two tiny docs PRs.