Add Docker development environment and CI/CD for Qiita article management

[Copilot]

Sets up containerized development environment and automated publishing pipeline for Qiita articles managed in this repository.

Changes

• Docker setup: Dockerfile with Node.js 20 and auto-install dependencies; docker-compose for dev container with Qiita CLI config volume mount
• CI/CD: GitHub Actions workflow publishes articles on push to main or manual trigger
• Project config: package.json with @qiita/qiita-cli, qiita.config.json for preview server (port 8888)
• Documentation: README with setup instructions and Docker usage examples

Usage

docker compose up
docker compose exec -it qiita-contents-dev bash
npx qiita preview  # Preview at localhost:8888

Articles in public/ auto-publish via GitHub Actions using QIITA_TOKEN secret.

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