Add hosted OpenAI-compatible gateway example

[haocn-ops]

This adds a small hosted OpenAI-compatible gateway example to the existing custom endpoint section.

API NODE uses the same openai_api_base path as other OpenAI-compatible endpoints, so the example is meant to make that hosted-gateway setup easier to recognize without changing the RubyLLM API surface.

Docs-only change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.03%. Comparing base (4942d6c) to head (d479715).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #761      +/-   ##
==========================================
- Coverage   87.05%   87.03%   -0.02%     
==========================================
  Files         119      119              
  Lines        5594     5594              
  Branches     1407     1407              
==========================================
- Hits         4870     4869       -1     
- Misses        724      725       +1     

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