[docs] Windows + LLVM instructions improvements

[merlinND]

• Clarify the supported VS versions. In at least one instance, VS 2022 worked whereas VS 2019 didn't.
• Repeat the suggested LLVM installation instructions from the DrJit docs
• Clarify how to proceed if LLVM is installed another way: set the DRJIT_LIBLLVM_PATH to point to the location of LLVM-C.dll.
  • It seems that installing with conda doesn't work: [❔ other question] LLVM backend is inactive #153 (comment)
  • Point out the behavior of environment variables on Windows vs new terminal sessions.
• Make sure there's only one set of setpath files in the actual build folder, not both build/ and build/Release

[njroussel]

Make sure there's only one set of setpath files in the actual build folder, not both build/ and build/Release

It's unfortunate, but I don't think there's a way we can get around this. There's some specific CMake limitations at play. I've at least updated the compilation instructions in the documentation with cc08904