fix: Convert Express quickstart from Docusaurus to Mintlify format

[brth31]

Description

• Added YAML frontmatter (title, description, sidebarTitle)
• Converted all Docusaurus syntax to Mintlify components:
  • :::note →
  • :::warning →
  • :::checkpoint →
  • ::::tabs/:::tab → /
  • :::details → with
• Added agent skills accordion for AI-assisted integration
• Fixed navigation display (was showing "index", now shows "Express")
• Fixed page rendering issue (content now loads properly)

Checklist

• I've read and followed CONTRIBUTING.md.
• I've tested the site build for this change locally.
• I've made appropriate docs updates for any code or config changes.
• I've coordinated with the Product Docs and/or Docs Management team about non-trivial changes.