This commit is a result of the discussion on rust-lang#128475 and incorporates
parts of rust-lang#109807 as well. This is all done as a new page of
documentation for the `wasm32-unknown-unknown` target which previously
did not exist. This new page goes into details about the preexisting
target and additionally documents the expectations for WebAssembly
features and code generation.

The tl;dr is that LLVM will enable features over time after most engines
have had support for awhile. Compiling without features requires
`-Ctarget-cpu=mvp` to rustc plus `-Zbuild-std` to Cargo.

Closes rust-lang#109807
Closes rust-lang#128475