[Docs]: Create standard usage guide for OpenAI integration

[rosspeili]

What needs to be fixed?

Once the openai adapter method (to_openai_tool) is merged into the core framework, we will have a glaring gap in our documentation. We need a corresponding docs/usage/openai.md guide for developers to reference.

The new guide should directly mirror the formatting and structure of the existing gemini.md and claude.md documentation, demonstrating exactly how to:

1. Initialize the global environment for openai.
2. Load a skill using the SkillLoader.
3. Format the skill using SkillLoader.to_openai_tool().
4. Pass the formatted tool to openai.chat.completions.create and handle the multi-turn agent execution loop.

Affected Page

docs/usage/openai.md (New File)