docs(readme): add Cloud Models Python usage and Cloud API example

[eden-chan]

Introduce a minimal, focused Cloud Models section for Python users.

Why

• Clarify how to run larger models via cloud offload while keeping local tooling.
• Document direct access to ollama.com’s API with a Python example.

What

• Add sign-in/pull/run flow for cloud models via local Ollama (automatic offload).
• Add example for direct cloud API with host="https://ollama.com" and OLLAMA_API_KEY header.
• List currently supported cloud model IDs.
• Keep examples concise; align with existing style and streaming pattern.

Scope

• Python-only; no JS/cURL duplication beyond a minimal tags listing.
• Docs-only change; no runtime or API modifications.

Validation

• Verified examples locally (streaming + basic chat).
• Ensured README ordering remains consistent.