Add instructions for using SDK without Emscripten

[mpwarres]

Context: https://github.com/proxy-wasm/proxy-wasm-cpp-sdk/pull/158/files#r1702364332

It should be possible to build against the C++ SDK using clang --target=wasm32-wasi as opposed to Emscripten. SDK documentation should include steps to do that.

Related: #169, #136, #145

[PiotrSikora]

My point was that we should drop Emscripten altogether, since it doesn't provide any value for non-browser environments.

[kyessenov]

Did anyone succeed in using this? This seems like much better UX for a small module.