docs: add Nebius Token Factory LLM service

[Arindam200]

This PR documents Pipecat’s Nebius Token Factory integration.

• Adds server/services/llm/nebius.mdx covering NebiusLLMService: overview, install (pipecat-ai[nebius]), env vars, configuration, usage (basic, settings, function calling), and notes on the developer role and model IDs.
• Registers the page under Server → LLM in docs.json.
• Lists Nebius in the LLM section of server/services/supported-services.mdx with the matching extra.

[markbackman]

Thanks for offering with the PR. We now auto-generate docs from PRs (and then human review with each release).

Closing in favor of #673.