docs: add governed OpenAI-compatible endpoint guide

[cerebrixos]

Summary

• add an Advanced guide for routing RubyLLM through governed OpenAI-compatible endpoints
• show Tuning Engines as a concrete Rails-friendly example using openai_api_base
• keep this as documentation only; no provider code or runtime behavior changes

Why

RubyLLM already supports custom OpenAI-compatible endpoints. This guide documents a production pattern for teams that want RubyLLM in application code while a gateway/control plane handles policy, audit, routing, quotas, and cost controls.

Validation

• Added one Jekyll docs page under docs/_advanced
• No code changes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.56%. Comparing base (6e634fd) to head (b324924).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #794      +/-   ##
==========================================
+ Coverage   87.55%   87.56%   +0.01%     
==========================================
  Files         125      125              
  Lines        5872     5872              
  Branches     1460     1460              
==========================================
+ Hits         5141     5142       +1     
+ Misses        731      730       -1     

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