[DO NOT MERGE] v2 preview

[mkozakov]

The pull request makes changes to the fern/v1.yml file, specifically the navigation section.

The changes include:

• Uncommenting the line - v2-beta under the audiences key, indicating that the v2-beta version is now part of the API specification.
• Adding a new section for V2 (beta) with skip-slug: true and defining its contents.
• Within the V2 (beta) section:
• Adding a subsection for /v2/chat with skip-slug: true and specifying its endpoints.
• Defining two endpoints for the /v2/chat section:
• POST /v2/chat with slug: chat-v2 and title: Chat Non-streaming.
• STREAM /v2/chat with slug: chat-stream-v2 and title: Chat Streaming.

[github-actions]

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