WEB: Restore website width and improve table of contents style
Follow up of #58791

This restore the website width (happy to make the content of PDEPs narrower, but it's a bit trickier so I'll leave it to another PR).

I also styled the table of contents more similar to the code blocks, which personally I think it looks a bit nicer.

Before:
![Screenshot at 2025-06-06 23-23-22](https://github.com/user-attachments/assets/7a025b7c-c5b6-4e6a-b921-627f4e3439cf)

After:
![Screenshot at 2025-06-06 23-22-37](https://github.com/user-attachments/assets/178642b6-8376-4992-bef0-224fddb2c97d)

CC @rhshadrach 
Thanks @datapythonista 