DOC: add Example sections to KernelFunction.from_prompt and invoke

[shoumikchakravarty-dev]

Add illustrative code examples to two public methods in KernelFunction:

• from_prompt: shows creating a prompt-based function and registering it with the kernel
• invoke: shows async invocation with KernelArguments using OpenAIChatCompletion

Both examples follow the existing Google-style docstring convention used throughout the module.

Motivation and Context

Please help reviewers and future users, providing the following information:

1. The from_prompt and invoke methods are the primary entry points for defining and running prompt-based functions in Semantic Kernel, yet their docstrings contain only a one-line description or a parameter list — no usage examples.
2. Developers exploring the Semantic Kernel Python SDK through IDE tooltips, help(), or generated API docs currently see no usage examples for from_prompt and invoke
3. Any scenario where a developer is getting started with Semantic Kernel's Python SDK and wants to understand how to define a prompt-based function and invoke it with arguments — without leaving their IDE or the API reference
4. No open issue. This is a proactive documentation improvement spotted while exploring the Python SDK codebase.

Description

This PR adds illustrative code examples to two public methods in
KernelFunction that currently have minimal docstrings:

• from_prompt: shows creating a prompt-based function and registering
  it with the kernel
• invoke: shows async invocation with KernelArguments using
  OpenAIChatCompletion

Both examples follow the existing Google-style docstring convention
used throughout the module. No logic changes.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible (Documentation only PR)
• I didn't break anyone 😄 (Documentation only PR)

[Copilot]

Pull request overview

Adds usage examples to two key public KernelFunction entry points in the Python SDK so developers can discover common workflows (creating prompt-based functions and invoking them) directly from docstrings / generated API docs.

Changes:

• Expanded KernelFunction.from_prompt docstring with an example that creates a prompt function and registers it with a Kernel.
• Expanded KernelFunction.invoke docstring with an async example invoking a prompt function using KernelArguments and OpenAIChatCompletion.

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