Add Respan to ecosystem docs

[drPod]

What this does

Adds Respan to the RubyLLM Ecosystem docs page. Respan is an LLM observability platform with a dedicated RubyLLM integration that works via its OpenAI-compatible gateway — no additional gems required.

Changes:

• New Respan section with gateway setup, key features, and code examples
• Added Respan to the OpenTelemetry section's list of compatible backends (it supports OTLP ingestion)
• Added Respan bullet to the "After reading this guide" list

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Performance improvement

Scope check

• I read the Contributing Guide
• This aligns with RubyLLM's focus on LLM communication
• This isn't application-specific logic that belongs in user code
• This benefits most users, not just my specific use case

Required for new features

N/A — documentation-only change.

Quality check

• I ran overcommit --install and all hooks pass
• I tested my changes thoroughly
  • For provider changes: Re-recorded VCR cassettes with bundle exec rake vcr:record[provider_name]
  • All tests pass: bundle exec rspec
• I updated documentation if needed
• I didn't modify auto-generated files manually (models.json, aliases.json)

AI-generated code

• I used AI tools to help write this code
• I have reviewed and understand all generated code (required if above is checked)

API changes

• Breaking change
• New public methods/classes
• Changed method signatures
• No API changes

[Copilot]

Pull request overview

Updates the RubyLLM Ecosystem reference docs to include Respan as an observability / gateway option, with setup guidance for routing RubyLLM traffic through an OpenAI-compatible gateway.

Changes:

• Adds Respan to the “After reading this guide” learning outcomes list.
• Updates the OpenTelemetry instrumentation section to list Respan as a compatible backend.
• Introduces a new “Respan” section describing features and providing RubyLLM configuration / usage examples.

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