[Obs AI Assistant] Add GPT-5-Chat ratings to the LLM performance matrix (#3816)

Closes elastic/obs-ai-team#374

This PR adds the performance ratings of `GPT-5` to the Observability AI
Assistant LLM performance matrix.

<img width="1288" height="166" alt="image"
src="https://github.com/user-attachments/assets/abb70386-28a9-4013-b571-a386a79eae3d"
/>

Author: viduni94

solutions/observability/llm-performance-matrix.md

@@ -37,6 +37,7 @@ Models from third-party LLM providers.
 | Google Gemini    | **Gemini 2.5 Flash**    | Excellent | Good | Excellent | Excellent | Excellent | Great | Good | Excellent |
 | Google Gemini    | **Gemini 2.5 Pro**    | Excellent | Great | Excellent | Excellent | Excellent | Great | Good | Excellent |
 | OpenAI    | **GPT-4.1**           | Excellent | Excellent | Excellent | Excellent | Excellent | Great | Good | Excellent |
+| OpenAI    | **GPT-5**           | Excellent | Excellent | Excellent | Excellent | Excellent | Excellent | Good | Excellent |
 
 
 ## Open-source models [_open_source_models]