Add Slidev beginner squat training deck

[kevinye8477]

Motivation

• Provide a minimal Slidev scaffold and a short beginner-friendly presentation titled 基礎深蹲訓練 入門 so the deck can be run with Slidev.
• Deliver a concise 5-page (intended) Chinese slide deck that teaches squat benefits, standard steps, common errors, and a starter weekly plan.
• Keep generated artifacts out of the repo by adding common ignore rules for Slidev builds and dependencies.

Description

• Add index.md containing the Slidev presentation content in Chinese covering benefits, 4-step technique, common errors with fixes, and a beginner training plan.
• Add package.json with dev/build/export scripts and a devDependencies entry for @slidev/cli to enable running and building the slide project.
• Add .gitignore with node_modules/, dist/, and .slidev/ to ignore build and dependency artifacts.

Testing

• Ran node -v and npm -v which succeeded (v22.21.1 and 11.4.2).
• Attempted npm install to validate Slidev setup but the registry returned 403 Forbidden when fetching @slidev/cli, so dependency installation failed.
• Verified the presentation source by printing index.md and ran a small python script to count slide blocks which reported 7 slide separators in the source.

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Codex Task