Add CONTRIBUTING.md and README.md for open source contributors

[sridhar-3009]

Task

The repo has no README or contributor guide. Before we can receive meaningful open source contributions, we need both.

README.md should include

• What the project is (one paragraph)
• Screenshot or GIF of the hero + a topic page
• How to run locally: git clone + python3 -m http.server 8080
• Project structure overview (which files do what)
• Link to the live site (if any)

CONTRIBUTING.md should include

• How to set up the dev environment (just clone + a local server)
• Code conventions:
  • Vanilla HTML/CSS/JS only — no npm, no build step, no frameworks
  • No emojis in HTML or canvas drawing code (use ASCII/unicode codepoints)
  • Canvas must use HiDPI scaling (devicePixelRatio)
  • All new text should follow the beginner-friendly writing style already used
• How to add a new SVG diagram to a topic page (reference the .diagram-box / .pkt pattern)
• How to submit a pull request
• Issue labels guide

Files to create

• README.md
• CONTRIBUTING.md

Difficulty: Beginner — no coding required, just writing