docs: add Tuning Engines custom endpoint example

[cerebrixos]

Describe the changes you have made:

Adds a docs-only example for using a Tuning Engines workspace through Open Interpreter's existing OpenAI-compatible custom endpoint support.

Reference any relevant issues (e.g. "Fixes #000"):

N/A

Pre-Submission Checklist (optional but appreciated):

• I have included relevant documentation updates (stored in /docs)
• I have read docs/CONTRIBUTING.md
• I have read docs/ROADMAP.md

OS Tests (optional but appreciated):

• Tested on Windows
• Tested on MacOS
• Tested on Linux

Docs-only change; runtime tests were not run.