Add DeepSeek V4 model docs

[peterschmidt85]

Summary

• add a DeepSeek V4 model page under examples/models/deepseek-v4
• add the DeepSeek V4 card to the examples landing page and the docs nav
• link the generic SGLang page to the DeepSeek V4 model page

Testing

• reviewed the staged diff for link, slug, and naming consistency
• uv run mkdocs build --strict

AI Assistance

• Assisted by Codex