Implement new dark theme design for documentation site

[Copilot]

This PR completely redesigns the Modulator library documentation site with a modern dark theme based on the provided mockup. The new design transforms the site from a basic Bootstrap-based layout to a sophisticated, custom-styled documentation experience.

Key Design Changes

Visual Design:

• Modern dark color scheme: Implemented gradients from #1d1d1b to #2b2a27 for the main background, with darker gradients for header (#181817 to #22211f) and sidebar (#1b1a18 to #242320)
• Professional typography: Added Google Fonts with Poppins for headings, Inter for body text, and JetBrains Mono for code blocks
• Card-based layout: Content sections now use semi-transparent backgrounds (rgba(255,255,255,0.04)) with subtle borders and shadows for depth
• Enhanced welcome section: Featured welcome heading and introductory paragraph in highlighted card sections to create visual hierarchy

Technical Improvements:

• Removed Bootstrap dependency: Replaced all Bootstrap classes with custom CSS for better performance and design control
• CSS custom properties: Implemented CSS variables for consistent theming throughout the site
• Flexbox layout: Modernized layout structure using flexbox instead of Bootstrap grid system
• Responsive design: Mobile-friendly layout that adapts seamlessly to different screen sizes

Interactive Elements:

• Smooth hover effects: Navigation items feature subtle animations with color transitions and transform effects
• Gold accent highlights: Links and interactive elements use #d4c389 accent color for better visual feedback
• Enhanced readability: Generous padding, proper line height (1.6), and carefully chosen text colors (#e8e4d8 primary, #bcb8aa secondary)

Files Modified

• docs/templates/SiteFrame.cshtml: Updated main layout template, removed Bootstrap dependencies
• docs/templates/Default.cshtml: Modernized page template with flexbox layout
• docs/templates/menu.cshtml: Redesigned navigation menu with custom styling
• docs/source/assets/main.css: Complete CSS rewrite with modern dark theme implementation

The new design closely matches the provided mockup and creates a professional, modern documentation experience that better represents the quality of the Modulator library.

Fixes #12.

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