chore: add README.md to repository root

[claude]

Problem

The repository has no README.md file at the root. This makes it difficult for:

• New contributors to understand the project
• Users to quickly grasp what Kubeasy is
• GitHub to display useful information on the repository homepage

Currently, all documentation lives in CLAUDE.md, which is intended for AI code assistants rather than human contributors.

Expected Content

A README.md should include:

• Project description and purpose (learning Kubernetes through interactive challenges)
• Tech stack overview (Next.js 15, TypeScript, Drizzle ORM, Better Auth, tRPC, Tailwind CSS)
• Prerequisites (Node.js, pnpm, PostgreSQL/Neon account)
• Quick start / local development setup
• Environment variables reference
• Available scripts (pnpm dev, pnpm build, pnpm check, etc.)
• Contributing guidelines
• License reference (LICENSE file exists)

Steps to Reproduce

1. Visit https://github.com/kubeasy-dev/website
2. Observe: no README displayed, just the file listing

Impact

• New contributors have no onboarding documentation
• Repository appears unmaintained or private-only
• GitHub search and discovery is reduced without a README

Suggested Fix

Create /README.md with content extracted and adapted from CLAUDE.md.

Note: The existing CLAUDE.md should remain as-is — it serves a different purpose (AI assistant context).