Spice up the project README, fill out more docs (#1010)

* Spice up the project README, fill out more docs

This is an attempt to spruce up wasmtime's README.md file as well as
fill out more of the missing documentation in the `docs/` folder.
There's still a long way to go but I hoped here to mostly move around
existing information and add new information. As always happy to have
feedback!

* Tweak CLI wording

* Remove no-longer relevant clause

* Update sandboxing docs

* Handle comments

docs/wasm-rust.md

@@ -28,10 +28,10 @@ Cross-compiling to WebAssembly involves a number of knobs that need
 configuration, but you can often gloss over these internal details by using
 build tooling intended for the WASI target. For example we can start out writing
 a WebAssembly binary with [`cargo
-wasi`](https://github.com/alexcrichton/cargo-wasi).
+wasi`](https://github.com/bytecodealliance/cargo-wasi).
 
 First up we'll [install `cargo
-wasi`](https://alexcrichton.github.io/cargo-wasi/install.html):
+wasi`](https://bytecodealliance.github.io/cargo-wasi/install.html):
 
 ```sh
 $ cargo install cargo-wasi
@@ -71,7 +71,7 @@ Hello, world!
 ```
 
 You can check out the [introductory documentation of
-`cargo-wasi`](https://alexcrichton.github.io/cargo-wasi/hello-world.html) as
+`cargo-wasi`](https://bytecodealliance.github.io/cargo-wasi/hello-world.html) as
 well for some more information.
 
 ## Writing Libraries