feat: modernize documentation UI and fix TOC auto-scroll

[Pranav-IIITM]

Overview

This PR modernizes the visual design of the Podman documentation site and fixes a usability bug in the Table of Contents (TOC) navigation on long doc pages.

What Changed

UI Modernization

• Glassmorphism — translucent backgrounds with backdrop-filter blur applied to the sidebar and TOC for visual depth
• Dark mode overhaul — replaced flat gray tones with a midnight-blue base (#0f172a) and high-contrast gradient accents
• Micro-animations — fade-in on page load, hover-lift on code blocks, slide transitions on nav links
• Typography & spacing — refined scale for improved readability across doc pages

TOC Auto-Scroll Fix

• Added tocAutoScroll.js, injected as a lightweight client module via docusaurus.config.js
• Uses a MutationObserver to detect when the --active class moves to a new TOC link
• If the active link falls outside the visible bounds of the TOC container, it's smoothly scrolled (scrollBy) back into view
• No changes to Docusaurus core behavior — implemented as an additive client module to stay upgrade-safe

Why

• The existing dark mode and static layout feel dated next to comparable documentation sites
• On long pages, the TOC's active-section indicator can scroll out of view with no way to auto re-center, forcing manual scrolling to stay oriented

Testing

• Verified dark mode contrast and glass effects across Chrome, Firefox, Safari
• Confirmed animations don't impact page load performance
• Tested TOC auto-scroll on long pages (e.g. docs/installation) in both light and dark mode
• Verified no regressions on mobile / narrow viewports

Screenshots

Before

Before.1.mp4

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After

After.1.mp4

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Related Issue

Closes #583