Extend the Miden SDK documentation in the source's doc comments

As I mentioned in https://github.com/0xMiden/docs/issues/177.

Let's extend the [Miden SDK docs](https://docs.rs/miden/latest/miden/) and directly link to it in the docs.

The benefits of the doc comments:
1. Available in the IDE to the users.
2. Available to the AI agents when they are exploring the `miden` dependency source code.
3. The code snippets in the doc comments are testable with `cargo test --doc` which guarantees that they are always up to date.
4. Lowest risk of being stale.

We can put "getting started" and "tuturial" type of docs on the SDK crates `lib.rs` level and extensive examples on the individual module (`note`, `account`, etc.) level in doc comments so they will be rendered first on the page in the `docs.rs` before the module API references. In the doc comments for the functions, types (structs, enums) we can also put any extra code examples we like.

I think we can put a reasonably large (few screens) amount of text even on the function/type level. The only limiting factor I see is the IDE UI usually renders it in a relatively small window so the user would have to scroll. So I think we should not limit ourselves in the amount of text we can put in. For the `lib.rs` and individual module level I don't think we should restrict ourselves at all.

/cc @Keinberger