[docs] Add httpx async example to Python README

[AryanSharma48]

Problem

The Python package supports asynchronous usage with httpx, but the main
Python usage documentation primarily demonstrates synchronous usage with
requests.

Users looking to integrate SmoothAPI into an async application would
benefit from a simple copy-pasteable example.

What needs to be done

Add an asynchronous httpx.AsyncClient example to the Python README.

The example should demonstrate:

• Creating an async client
• Configuring SmoothAPI
• Making an asynchronous request
• Properly awaiting the request

Acceptance Criteria

• An httpx.AsyncClient example is included
• The example uses current SmoothAPI APIs
• The example is valid Python
• The example demonstrates async/await
• Documentation clearly explains when the async example should be used

Files to look at

• packages/smooth-api-py/README.md
• README.md

Prerequisites

Basic Python and basic async/await knowledge.

Notes

Keep the example concise and consistent with the existing documentation.