Mistral Doc - Enhance sidebar list heading text color

Currently, the sidebar list heading text color is very dark and hard to see deu to it text color. 

<img width="362" height="813" alt="Image" src="https://github.com/user-attachments/assets/c448c022-c3e9-418c-9697-7b10b483ab40" />


Proposed Improvement

<img width="342" height="905" alt="Image" src="https://github.com/user-attachments/assets/93286597-db57-4535-8f58-f96e569d1ee5" />
