docs: overhaul SVG diagrams and add style guide

docs: overhaul SVG diagrams and add style guide

- Scale all SVG diagrams to larger dimensions (640x480/640x520) for
  improved visibility
- Enhance shading diagrams with detailed annotations: Lambert cosine
  law angle examples, ambient light comparison panels, distance
  attenuation curve plot
- Create doc/style.css and link from all pages to consolidate shared
  SVG theme and responsive media styles
- Remove completed TODO item for coordinate system diagram
- Fix internal link in index.org to use simpler anchor syntax