[Docs] Update installation guide

[alexanderkim11]

📚 Documentation

It seems like section 2.3 Installation in the README is out of date, as it says to checkout the testnet-beta branch of the repo, and doesn't even mention that you can just use cargo install instead of cloning from source.

Also there is an additional step when trying to install snarkOS on Windows. Users need to install C++ Clang tools for Windows via the Visual Studio Installer, then set the LIBCLANG_PATH environment variable to the location of the installed libclang.dll file. By default, I believe this should be in: Program Files (x86)\Microsoft Visual Studio\2022\BuildTools\VC\Tools\Llvm\x64\bin