Enhance the retro theme's API and documentation pages to better mimic
the Windows 98 aesthetic:
- Convert the docs header into a full-width grey window with black text.
- Style section card headings (`h2`) as classic blue gradient title bars, complete with a mock close button ("✕").
- Adjust margins to make top-level headings flush with window edges.
- Hide the kicker element in the docs header.
Daniel Legt
48d3c0475f