Make examples in design section compile

[lenguyenthanh]

PR Checklist (not all items are relevant to all PRs)

• Added unit-tests (for runtime code)
• Added bootstrapped code + smoke tests (when the rendering logic is modified)
• Added build-plugins integration tests (when reflection loading is required at codegen-time)
• Added alloy compliance tests (when simpleRestJson protocol behaviour is expanded/updated)
• Updated dynamic module to match generated-code behaviour
• Added documentation
• Updated changelog

[kubukoz]

Thanks! It'd be cool to have these actually go through mdoc, but I don't think it's necessary. The design pages are more like blogposts, which are often difficult to mdoc-ify because we keep rewriting some definitions.