fix(seo): correct OG image URL, fix homepage title, and add AI-friendly robots.txt

- Change og:image to absolute URL (https://33jsconcepts.com/og-image.png)
- Add thumbnails config and article:author to docs.json
- Fix homepage title from '33 JavaScript Concepts' to 'Learn JavaScript' to prevent duplication
- Create robots.txt allowing AI search engine bots (GPTBot, ClaudeBot, PerplexityBot)