Code excerpts: use new tooling and infra

[chalin]

We currently have two mechanisms for dealing with code fragments: the old approach (using Jekyll include on individual fragments) and the new (where fragments are excerpt from fully tested apps).

The Get Started codelab and the Building Layouts tutorial use the new mechanism. This issue will be used to track the updates of remaining pages.

• Get Started codelab
• Layouts tutorial
• Layouts in Flutter
• Animations
• i18n
• development/platform-integration/c-interop.md