Add OrcaRouter integration page

[jinhaosong-source]

Summary

• Adds an integration page for OrcaRouter, an OpenAI-compatible model routing gateway
• OrcaRouter exposes chat models from providers such as OpenAI, Anthropic, Google, DeepSeek, and Qwen behind a single endpoint and API key, addressed with a provider/model namespace (e.g. openai/gpt-4o-mini)
• The orcarouter-haystack package provides the OrcaRouterChatGenerator component
• PyPI: https://pypi.org/project/orcarouter-haystack/
• Source: https://github.com/deepset-ai/haystack-core-integrations/tree/main/integrations/orcarouter

Follow-up to the merged integration (deepset-ai/haystack-core-integrations#3436), as requested by @anakin87.

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