Add root-level simple README overview

[Copilot]

This change addresses the request for an additional README by adding a short, plain-language entry point for new readers. It complements the existing full documentation with a quick orientation page.

• What changed

  • Added README_SIMPLE.md at repository root.
  • Documented a minimal project summary, purpose of the simplified doc, and pointers to canonical docs.

• Content focus

  • Briefly explains what PipeCD is in non-specialized language.
  • Clarifies when to use this file vs. the main README.md.
  • Links readers to deeper documentation under docs/.

• Example (new file content shape)

  # PipeCD (Simple README)
  
  ## What this project is
  PipeCD is a continuous delivery tool.
  
  ## Why this file exists
  The main README is detailed; this is a quick overview.
  
  ## Where to go next
  - README.md
  - docs/

[Ayushmore1214]

@copilot also add a readme.md file in this folder and epxlain your each and every chnages in this pr nd why it is needed and all