[New docs] Integrate <Demo /> component with code blocks

[tkajtoch]

Summary

We should allow defining interactive code examples using the simple code block syntax when the example is single-file and doesn't need the flexibility and additional features of the <Demo /> component.

Syntax

Interactive code examples should be based on regular code block syntax with demo metastring added to differentiate from "static" code blocks.

Note: ignore // characters in the example below

// ```tsx demo
// <EuiButton>Button</EuiButton>
// ```