Add openrouter conversation component docs

[jjcollinge]

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Description

Add docs for dapr/components-contrib#4334

Issue reference

[nelson-parente]

Pre-review for @marcduiker handoff: LGTM with nits.

• New OpenRouter conversation component page looks correct: key metadata fields, cacheTTL, siteURL/siteTitle for attribution, and the data/components YAML entry with since: "1.18".
• Nit: the DCO checklist items are all unchecked. The author should sign their commits per Dapr contributing guidelines. Marc, worth confirming DCO is satisfied before merging.
• Nit: model field shows openai/gpt-4o-mini as example in YAML but the description says default is also openai/gpt-4o-mini — consistent, good.
  Nothing blocking if DCO is satisfied — Marc, your call.

[marcduiker]

LGTM