docs: cosmic theme revamp for the documentation site

[reidmorrison]

Revamps the documentation site (logger.rocketjob.io) to match the new v5.0.0 key art — a deep-space theme replacing the long-standing black→red header, green headings, and light code blocks.

Changes

• Header: starfield banner (cropped from the key art, darkened for legibility), glossy two-tone gradient title (blue "Semantic", orange "Logger"), and the new chrome-and-red rocket icon.
• Palette: section headings green → cosmic blue; links aligned to cosmic blue; active/hover nav buttons → flame orange (echoing the rocket exhaust).
• Code blocks: dark navy surface with a fully remapped Pygments/Rouge token palette (orange keywords, green strings, sky-blue builtins, gold numbers, violet method/class names). Rouge on GitHub Pages emits these same classes, so it matches live.
• Icons: multi-resolution favicon.ico and apple-touch-icon.png regenerated from the chrome rocket, flattened onto navy (iOS renders transparency as black). Old red-rocket art retired.
• Landing page: full key art added as a compact centered hero card.
• Tagline: trimmed the redundant "Semantic Logger" prefix now that the name is in the gradient title.

Only the Jekyll layout, two stylesheets, index.md, and image assets change — no gem code touched.

Verification

Built locally with jekyll build and screenshotted the landing page and a code-heavy page (appenders) in headless Chrome: header, gradient title, nav, hero, headings/links, and dark syntax highlighting all render correctly.

🤖 Generated with Claude Code