improve/document examples in `wasmtime/cranelift/jit/`

Hi, I just came from https://github.com/bytecodealliance/cranelift-jit-demo which is nice, documented and pretty straightforward (although has some [todo comments left there 3-4 years ago](https://github.com/bytecodealliance/cranelift-jit-demo/blob/2e1fd35e78bb64c0df0a835cc6199b377fd0fb9a/src/jit.rs#L53-L55))

#### Improvement

Add more examples of different complexity (basic to medium and maybe some complex ones), document them with explanations of what's what (like it is done in [cranelift-jit-demo](https://github.com/bytecodealliance/cranelift-jit-demo))

#### Implementation

Think of some simple examples and use-cases that incorporate jit, it could be something like compiled regex or other type of state machine pattern, then add examples commenting every jit and cranelift-specific step to avoid confusion and provide guidance.

Show how to define functions, work with data, leave useful notes to external resources (at least to Wikipedia).

#### Benefit

Newcomers like me will be able to understand what is going on there and how they can use cranelift jit to build their own ~~blazingly fast~~ & feature packed language.

p.s. now I'm trying to explore cranelift-jit-demo and it is looking good so far, I like it.