docs: adding Perplexity AI use case document in tabby-docs (#3678)

* docs(perplexity): add documentation for Perplexity AI models and API usage

* chore: Update website/docs/references/models-http-api/perplexity.md

Co-authored-by: Meng Zhang <[email protected]>

---------

Co-authored-by: Meng Zhang <[email protected]>

Author: Sma1lboy

website/docs/references/models-http-api/perplexity.md

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+# Perplexity AI
+
+[Perplexity AI](https://www.perplexity.ai/) is a company that develops large language models and offers them through their API service. They currently provide three powerful Llama-based models: [Sonar Small (8B)](https://docs.perplexity.ai/guides/model-cards#supported-models), [Sonar Large (70B)](https://docs.perplexity.ai/guides/model-cards#supported-models), and [Sonar Huge (405B)](https://docs.perplexity.ai/guides/model-cards#supported-models), all supporting a 128k context window.
+
+## Chat model
+
+Perplexity provides an OpenAI-compatible chat API interface. The Sonar Large (70B) and Huge (405B) models are recommended for better performance.
+
+```toml title="~/.tabby/config.toml"
+[model.chat.http]
+kind = "openai/chat"
+model_name = "llama-3.1-sonar-large-128k-online"  # Also supports sonar-small-128k-online or sonar-huge-128k-online
+api_endpoint = "https://api.perplexity.ai"
+api_key = "your-api-key"
+```
+
+## Completion model
+
+Perplexity currently does not offer completion-specific API endpoints.
+
+## Embeddings model
+
+Perplexity currently does not offer embeddings models through their API.