PR: Enhance Docsify UI with Gradient Background, Vertical Heading Borders, and Back-to-Top Improvements

Description

This PR improves the overall look and usability of the Learn PostgreSQL Docsify site by introducing several UI enhancements.

Changes Include

1. Gradient Background

   • Added a soft blue-purple diagonal gradient to the page background (body).
   • Optional subtle radial overlay for depth and visual appeal.

2. Heading Styling

   • Added a purple vertical border on the left of all headings (h1, h2, h3) for a modern doc-style look.
   • Consistent spacing and font-weight adjustments for readability.
   • Optional gradient and glow effects for future enhancement.

3. Back-to-Top Button

   • Enhanced hover and active states with gradient and shadow.
   • Smooth scroll behavior implemented for better UX.
   • Reduced motion support for accessibility.

4. PDF Links

   • Updated styling for links ending in .pdf for better visibility with subtle hover effect.

5. Typography & Layout

   • Consistent font, spacing, and line-height for all markdown content.
   • Improved padding and alignment for headings and sections.

Impact

• Gives the site a modern, polished, and visually appealing look.
• Improves navigation and readability, especially for long pages.
• Ready for further enhancements like dark mode or heading-level gradients.

Notes

• All changes are purely CSS/JS — no modifications to markdown content.
• Fully compatible with the current Docsify Vue theme.