feat: Separate SEO content to /about page (best practice)

- Remove header from main page for clean editor-only interface
- Create /app/about/page.tsx with comprehensive SEO content (1000+ words)
- Add About link next to 'Next-AI-Drawio' title in chat panel
- Add GitHub icon link to /about page navigation
- Update sitemap.ts to include /about page (priority: 0.8)

SEO improvements following industry best practices:
- Separate marketing content from app interface (Figma/Canva/Miro approach)
- Server-rendered /about page for optimal crawlability
- Clean URL structure for better internal linking
- Multiple indexable pages for broader keyword coverage
- Proper semantic HTML: H1, H2, H3, article, section tags
- 1000+ words of keyword-rich content

/about page includes:
- AI diagram generator overview with value proposition
- 6 detailed feature sections (AI creation, AWS diagrams, image replication, etc.)
- 3 popular use cases (AWS architecture, flowcharts, system design)
- Step-by-step usage guide (4 steps)
- Benefits section (save time, precision, free, privacy)
- Clear call-to-action with link back to editor
- GitHub link in navigation for social proof

This follows Google-approved architecture and avoids hidden content penalties.

app/sitemap.ts

@@ -8,5 +8,11 @@ export default function sitemap(): MetadataRoute.Sitemap {
             changeFrequency: 'weekly',
             priority: 1,
         },
+        {
+            url: 'https://next-ai-drawio.jiang.jp/about',
+            lastModified: new Date(),
+            changeFrequency: 'monthly',
+            priority: 0.8,
+        },
     ]
 }