Update HTML Classes content with JS and BEM examples

[darshan-totagi]

Enhanced HTML Classes section with syntax, same-class reuse, JavaScript usage, and BEM naming examples

• Added comprehensive explanations on HTML class syntax and usage.
• Included new examples showing multiple and reusable class combinations.
• Added JavaScript snippets demonstrating dynamic class selection and manipulation.
• Introduced BEM (Block Element Modifier) naming convention with visual examples.
• Improved readability and styling of all code boxes using a dark theme for better contrast.
• Ensured consistent formatting across HTML, CSS, and JS examples.
• Retained existing navbar and sidebar layout for seamless integration.

[github-actions]

Message that will be displayed on users' first pull request