feat(website): refine gradient overlay and adjust layout for responsiveness

[SteveLin100132]

This pull request focuses on improving the visual presentation and user experience of the landing page and documentation for the Notion Chart Generator project. The main changes include enhancements to the README.md for clearer branding and demo visuals, as well as UI adjustments in key website components to refine layout and responsiveness.

README and Documentation Enhancements:

• Added a new banner image and updated the logo in README.md for improved branding and first impression. Also, restructured the introductory section to use semantic HTML and included quick access links for website, documentation, and author info.
• Updated the demo image in README.md to showcase basic features, replacing the previous usage demo for better clarity.

Website UI Improvements:

• In HeroSection.tsx, made the white gradient overlay at the bottom visible only on medium screens and above, and increased its height for a smoother fade-out effect. Also, changed the main content container to use h-fit for better layout flexibility.
• Hid the app preview image in the hero section on small screens to improve mobile responsiveness and visual hierarchy.
• In Footer.tsx, improved the alignment of the heart icon by making it inline, and simplified the container for better styling consistency.
• In HowToUseSection.tsx, added a relative z-10 class to the button container to ensure proper stacking and visibility over background elements.