Revise README and enhance index.html with improved UI and functionality

[vanshangra]

This PR delivers a major UI/UX redesign and code improvement for the Weather App.

Changes Made

Modernized the UI:

• Redesigned the layout for a cleaner, more professional look.
• Weather details are now displayed in a grid with clear headings and icons.
• The current weather card and 5-day forecast chart are shown side by side on desktop, stacked on mobile.
• Improved color scheme, spacing, and card styling for readability and aesthetics.
• Updated subtitle and footer for clarity.

API Usage Update:

• Switched from the deprecated /forecast/daily endpoint to /forecast (3-hour intervals).
• Implemented daily grouping/averaging in the frontend to display 5-day summaries.

Responsiveness:
Enhanced mobile and desktop experience with responsive flex/grid layouts.

Documentation:
Updated the README with usage instructions, API key info, and a project screenshot.

Result
The app now provides a much cleaner, more user-friendly experience, with accurate weather data and a visually appealing, responsive interface.
All core features remain, and the app is compatible with the latest OpenWeatherMap API.