[clang] [analyzer][docs] CSA release notes for clang-20 (PR #124798)

[Balazs Benics via cfe-commits]

steakhal wrote:

![page1](https://github.com/user-attachments/assets/5f0c96be-0a83-44e8-a29f-a03cacf7ab50)
![page2](https://github.com/user-attachments/assets/09db787a-8ca4-48b7-b1fd-725b70158464)


https://github.com/llvm/llvm-project/pull/124798