docs: add Gemini provider example and README entry

[Frankie-Xu]

Summary

• Add examples/providers/google_demo.py matching the Anthropic demo: create_client(llm="google", embedder="openai:text-embedding-3-small"), with hyperextract[google] and GOOGLE_API_KEY / GEMINI_API_KEY.
• List the demo in both examples READMEs.
• README + README_ZH: Gemini he config snippet, gemini-3.8-flash model-table row, and a Provider-notes pairing paragraph.

Closes yifanfeng97#156

Out of scope

• Changing client.py or config presets
• Rewriting docs/*/concepts/provider-system.md
• Version / news edits

Test plan

• python -m py_compile examples/providers/google_demo.py
• Default model string matches gemini-3.8-flash in client.py

  

[Frankie-Xu]

Opened on the fork by mistake. Review is at yifanfeng97#157 — closing this.