Update demo apps to adhere to vector branding guidelines

[amrit110]

This pull request focuses on improving documentation clarity, modernizing the UI, and unifying branding across the aieng-auth project and its demo applications. The most significant changes include a major rewrite and simplification of the README.md, UI/UX updates to both the React and Next.js demo apps, and consistent branding updates throughout the project.

Documentation improvements:

• Major rewrite and simplification of README.md:
  • Clarifies that the package is for Vector internal web applications.
  • Provides clearer setup instructions for both admins and developers.
  • Streamlines feature and package descriptions, and improves quick start and security documentation. [1] [2] [3] [4]
• Updates to Google Cloud setup instructions in SETUP_GCLOUD.md for consistent naming (aieng-auth demo) and clarity. [1] [2]

Branding and naming consistency:

• Updates all references from "CyberArk Auth Demo" or "AIEng Auth Demo" to "aieng-auth demo" in HTML titles, documentation, and UI text. [1] [2] [3] [4] [5]

UI/UX and style enhancements:

• Modernizes the color palette and font:
  • Applies a new gradient background and accent color (#EB088A and #313CFF) in both React and Next.js demo apps.
  • Switches to the 'Open Sans' font for improved readability and a more modern look. [1] [2] [3] [4] [5] [6] [7]
• Simplifies and clarifies demo application UI text:
  • Removes emoji and redundant explanations from headings and status messages.
  • Streamlines protected page messaging and removes extra instructional cards from dashboard pages. [1] [2] [3] [4] [5] [6] [7] [8] [9]

Demo application improvements:

• Updates demo application instructions in the documentation for clarity and accuracy.
• Unifies authentication flow descriptions and feature lists across both React and Next.js demos for a more consistent developer experience. [1] [2] [3] [4] [5]

These changes collectively make the project easier to understand, more visually appealing, and more consistent for both users and contributors.