* doc: add a page listing supported proposals
This adds a table showing Wasmtime's support for various WASI proposals,
much like the one available for WebAssembly proposals. This change is
related to [#2423], which provides guidelines for implementing WASI
proposals but was never merged.
[#2423]: https://github.com/bytecodealliance/wasmtime/pull/2423
* review: remove phase-gating sentence
