Start a high-level architecture document for Wasmtime (#3019)

* Start a high-level architecture document for Wasmtime

This commit cleands up some existing documentation by removing a number
of "noop README files" and starting a high-level overview of the
architecture of Wasmtime. I've placed this documentation under the
contributing section of the book since it seems most useful for possible
contributors.

I've surely left some things out in this pass, and am happy to add more!

* Review comments

* More rewording

* typos

crates/jit/Cargo.toml

@@ -8,7 +8,6 @@ license = "Apache-2.0 WITH LLVM-exception"
 categories = ["wasm"]
 keywords = ["webassembly", "wasm"]
 repository = "https://github.com/bytecodealliance/wasmtime"
-readme = "README.md"
 edition = "2018"
 
 [dependencies]

crates/jit/README.md

@@ -1,10 +0,0 @@
-This is the `wasmtime-jit` crate, which contains JIT-based execution
-for wasm, using the wasm ABI defined by [`wasmtime-environ`] and the
-runtime support provided by [`wasmtime-runtime`].
-
-Most users will want to use the main [`wasmtime`] crate instead of using this
-crate directly.
-
-[`wasmtime-environ`]: https://crates.io/crates/wasmtime-environ
-[`wasmtime-runtime`]: https://crates.io/crates/wasmtime-runtime
-[`wasmtime`]: https://crates.io/crates/wasmtime