[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: https://github.com/mitsuba-renderer/mitsuba3/issues/153#issuecomment-1199722702
  • [ ] 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