Improve tutorial.md to reference freenet-email app directly

[sanity]

Currently the examples in the tutorial leave out parts of the code:

struct Posts(...)

impl Posts {
  fn add_post(&mut self, post: Post) { ... }
}

struct Post(...)

The tutorial should instead reference a working example, most likely the freenet-email-app. mdbook has useful functionality for including portions of files from elsewhere in the repo. Using this will ensure that the documentation stays in sync with the code.