[openmp] [libc] [clang] [mlir] [llvm] [compiler-rt] [analyzer][docs] Update the release notes for llvm-18 (PR #76446)

[Balazs Benics via cfe-commits]

steakhal wrote:

Introduced subsub-sections and ordered them and their content by expected relevance.
The rule of thumb was:
 - released stuff comes first, then alpha. etc.
 Also, checkers are always fully named, and named by their user-facing names.
 
 Now the html looks like this: 
![image](https://github.com/llvm/llvm-project/assets/6280485/5f87a250-98a4-4860-a107-acd754aa7d1f)


I'm pretty happy with the result.

https://github.com/llvm/llvm-project/pull/76446