[analyzer][docs] Update the release notes for llvm-18 (#76446)

This PR prepares the release notes of the Clang Static Analyzer for the
llvm-18 release branch, due in about a week.
See the regular [release schedule](https://llvm.org/docs/HowToReleaseLLVM.html#annual-release-schedule).

This patch was written after examining the relevant Static Analyzer
commits since the last release.
Have a look at the commits, and provide feedback if I missed anything
interesting.
Note that the release notes is not meant to be an exhaustive list of the
changes, but rather a curated list of the relevant changes that might
interest our stakeholders, such as tool vendors based on top of CSA or
users with custom checkers.

See the relevant commits by using this command:
```
git log --oneline llvmorg-18-init..llvm/main   clang/{lib/StaticAnalyzer,include/clang/StaticAnalyzer} | grep -v NFC | grep -v -i revert
```

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