Add CI to the book (#2027)

## Motivation and Context

Closes https://github.com/awslabs/smithy-rs/issues/2004



## Description

Run `mdbook test` over the `design` folder.

## TODO

- [x] Ignore the RFC sections using `ignore` tag on the code blocks.
- [ ] Fix the remaining examples.
- [x] Ensure local `rust-runtime` dependencies are being used.

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Signed-off-by: Daniele Ahmed <ahmeddan@amazon.de>
Co-authored-by: 82marbag <69267416+82marbag@users.noreply.github.com>