feat: Create initial AI content web platform - #56

This commit establishes the foundational structure for an AI learning web platform.

Key features implemented:
- Basic HTML structure for a homepage (`index.html`) and four content sections:
    - Machine Learning (`machine_learning.html`)
    - Natural Language Processing (`natural_language_processing.html`)
    - Computer Vision (`computer_vision.html`)
    - AI Ethics (`ai_ethics.html`)
- Initial placeholder content and definitions added to each section page.
- Navigation bar on the homepage linking to all sections.
- Each section page includes a navigation link back to the homepage.
- Basic CSS styling (`style.css`) applied for a clean and readable layout, including header, footer, navigation, and content area styling.
- JavaScript file (`script.js`) linked, though no active scripts are implemented yet.
- All pages are interlinked correctly and reference the shared CSS and JS files.