Refactor and enhance custom UI elements

This commit modernizes the custom top banner and floating buttons by refactoring their styling and improving their behavior. Key changes include: - Moved inline styles from overrides/main.html to the dedicated custom.f7ec4df2.min.css file for better organization. - Replaced the image icon in the "Ask AI" floating button with a Material Design SVG icon (auto_awesome). - Implemented smoother CSS transitions for hover effects on the banner and floating buttons. - Enhanced responsiveness of the banner (now stacks on mobile) and floating buttons (adjusted sizes and positions for mobile) to ensure they adapt well across device sizes. - Improved light/dark mode integration by using MkDocs Material theme variables for colors and shadows, ensuring visual consistency. - Confirmed z-index and CSS specificity to minimize conflicts with other page elements like IconSearch or tabbed content.