docs: improve Pay-Then-Vault documentation aesthetics

[AbhishekChorotiya]

Summary

• improve the visual hierarchy and layout across Pay-Then-Vault Web, Mobile, and Server Setup
• preserve the existing landing-page images and present them in cleaner, more readable layouts
• render real integration workflows as native GitBook steppers across Web, Android, iOS, React Native, and Flutter
• strengthen customization, widget, troubleshooting, error-code, and reference-page hierarchy
• add consistent information, warning, danger, demo, and success callouts
• keep a standard content width with a fixed right-side On this page outline where page headings are available

Scope

Only Payment Experience > Pay-Then-Vault > Web, Mobile, and Server Setup are changed. Payment Flows and every other documentation section remain untouched.

Content and image safety

• no instructional prose was added, removed, or rewritten
• no code example was changed
• all original image references and links are preserved
• native step numbers replace redundant typed top-level prefixes while preserving the visible sequence

Validation

• GitBook preview and SentinelOne checks pass
• all 29 Mobile and Server Setup preview routes return successfully
• Playwright verified representative Android, iOS, React Native, Flutter, Mobile landing, and Server Setup pages
• inspected native steppers, nested Swift and SwiftUI tabs, callouts, tables, code blocks, preserved landing images, and fixed desktop outlines
• checked representative integration and customization pages at desktop and mobile viewport widths
• validated frontmatter, GitBook block nesting, HTML details, code fences, and diff whitespace
• compared normalized prose, fenced code, HTML pre blocks, URLs, and image references against the pre-change revision with no differences