[llvm] [docs][CMake] Add CMakePreset, improve VS documentation (PR #84760)

[Tobias Hieta via llvm-commits]

tru wrote:

> Thank you for the improvements here, it's really appreciated! Note, as a long-time user of this functionality, there is actually one thing that drives me bonkers over all other usability issues: because of the way the monorepo is set up, CMake support within Visual Studio is excellent for LLVM and terrible for every other project. For example, "go to definition", the solution explorer, etc are all focused solely on the `llvm` folder and have no knowledge of the `clang`, et al folders in the monorepo. The IDE experience is very much hampered by this, and anything we can do (either in terms of manual interventions, of which I've never found one, or in terms of presets, layout changes, etc) to improve this would be a _huge_ productivity boost. It's the _one_ feature I miss on a daily basis from using CMake to generate a solution file.

I wonder why this is, do you open the root `llvm-project` and not the `llvm` subfolder?

https://github.com/llvm/llvm-project/pull/84760