feat: Add Nebius Token Factory provider documentation

[Arindam200]

This PR adds documentation for integrating and authenticating with Nebius AI Studio as a provider. It walks users through setting up their Nebius account, adding API keys, and selecting models via the CLI.

Closes: #2933

Key Additions:

• Added Nebius AI Studio section to the provider documentation
• Included detailed setup steps and example CLI commands for authentication
• Explained how to list and select Nebius models using the /models command

Related Work: Nebius AI Studio models are being added in a separate PR on models.dev: https://github.com/sst/models.dev/pull/271