docs: update Developer Build an Inference Engine

[0xHieu01]

Pages Build an Inference Engine: https://jan.ai/developer/build-engine/ Using the OpenAI engine implementation as an example

Why

• OpenAI engine extension implementation doesn't rely on module.ts and is a good representation of how we intend the SDK to be used
• There are still a lot of gotchas (things we intend to change) but it is a more "ready" example than other extensions.

Success Criteria Content that should be covered

• [x] In this tutorial, you will learn how to build an inference engine on Jan
• [x] Developer docs on how to extend Jan. These pages are about what people can build. It should sufficiently outlining the overarching architecture to empower developers to effectively develop upon it.

Tasklist:

• [ ] Understand code / intentions
• [ ] clone openai into an addn extension...
• [x] Update extension-template boilerplate: https://github.com/janhq/extension-template
• [ ] Figure out development flow: how to debug extensions, how to import, how to delete, etc.
• [x] Write the docs
  • [x] Page: Your First Extension
  • [ ] Page: Anatomy of an Extension
  • [ ] Page: Package your Extension
  • ^ Page sequence is just a suggestion

Additional context Inspo: https://code.visualstudio.com/api/get-started/your-first-extension