docs: Revamp System Overview, Marketing Usecase, and Get Started Sections

[selcukemiravci]

This PR introduces several major updates and reorganizations in our documentation:

• System Overview Page:

  • Updated the content with a more concise and context-focused introduction.
  • Revised the architecture breakdown for clarity.

• Marketing Usecase Page:

  • Redesigned and improved the content layout.
  • Integrated new Mermaid diagrams to better illustrate the marketing agent process.

• Get Started Section:

  • Added new images and reorganized content for better readability.
  • Moved and renamed files to create a more logical documentation flow.

• Security Fixes:

  • Fixed security vulnerabilities by upgrading estree-util-value-to-estree and image-size.