Restructure README to be clearer and communicate the USPs

[marc-thieme]

The README needs a minor update to emphasize the package's unique selling points (USPs) and the design considerations:

1. Ergonomic Syntax: The frame functions offer a concise and user-friendly syntax, with the #divide() feature being particularly noteworthy.
2. Customization: Creating custom frames with unique supplements and colors is straightforward and quick, covering most use cases.
3. Predefined Styles: The default styles are visually appealing.
4. Extensibility: The interface for custom styling functions is easy to understand and use and allows for arbitrary styles.
5. Integration: The package integrates seamlessly with existing Typst infrastructure, supporting features like page breaks and outlines by wrapping frames in a single figure.

[marc-thieme]

Also make custom styling more accessible. Link to the source where the default styles are defined and encourage people to copy them.