Simplify quickstart page and streamline documentation structure

[mintlify-development]

Original Instruction: Simplify the quickstart page

This PR simplifies the quickstart page to improve the onboarding experience for new users. The changes include reorganization and streamlining of content across multiple documentation sections:

Key Changes:

• Simplified the quickstart.mdx page for better readability and user flow
• Updated related documentation files to maintain consistency
• Reorganized content structure in advanced, api-playground, and integration sections
• Ensured documentation alignment across authentication, analytics, and SDK pages

Modified Files:

• Core changes: quickstart.mdx, development.mdx, migration.mdx
• Advanced documentation: Multiple files in user-auth, widget, and subpath sections
• API Playground: Configuration and OpenAPI setup documentation
• Integration guides: Analytics, Privacy, and SDK documentation
• Settings and Components: GitLab settings and Mermaid diagram documentation

[hahnbeelee]

closinggg