Redesign documentation with OpenRouter-inspired theme

- Implement modern minimal design with purple accent colors
- Fix dark theme to use consistent pure black tones
- Add collapsible sidebar categories with +/- indicators
- Add 'On this page' TOC component in right sidebar
- Add Previous/Next page navigation at bottom
- Center content layout for better readability
- Change default link colors to white with purple hover
- Hide sidebar sub-navigation for cleaner structure
- Remove forward/backward navigation buttons
- Switch from Plus Jakarta Sans to Inter font
- Update landing page styling to match new aesthetic

New files:
- theme/sidebar.js: Collapsible category functionality
- theme/toc.js: Table of contents component
- theme/pagenav.js: Page navigation component

Author: Surendra Raika

book.toml

@@ -14,3 +14,4 @@ git-repository-url = "https://github.com/RaikaSurendra/go-design-patterns"
 edit-url-template = "https://github.com/RaikaSurendra/go-design-patterns/edit/master/{path}"
 no-section-label = true
 additional-css = ["theme/css/custom.css", "theme/css/landing.css"]
+additional-js = ["theme/sidebar.js", "theme/toc.js", "theme/pagenav.js"]