[Documentation] Clear information about supported WebAssembly features

[mijamo]

I think it would help users to have some clear information directly accessible about:

• What version of WebAssembly is currently supported and if there is any feature that is not supported (right now it is quite straightforward but better having it displayed clearly)
• Which of the WebAssembly proposals are supported (from https://github.com/WebAssembly/proposals)
• Which WASI snapshop is supported
• Which WASI proposals (from https://github.com/WebAssembly/WASI/blob/master/docs/Proposals.md) are supported if any

Right now it requires quite a bit of search to check if things like reference types or SIMD are supported (or even multi value return which is not in WA 1.0)