Add 4 new JAMstack themes: Gatsby WordPress Blog and 3 Next.js portfolios

[tomcomtang]

This pull request adds four new open-source themes to the collection:

�� New Themes Added

1. Gatsby WordPress Blog

• SSG: Gatsby
• CMS: WordPress
• Features: Modern blog theme integrating WordPress and Gatsby, supporting both remote WordPress content and local config file modes
• Comments: Optional Giscus integration
• Demo: https://wordpress-gatsby-demo.example.com

2. Next Minimal Gallery

• SSG: Next.js
• CMS: No CMS
• Features: Minimalist image gallery theme focused on photo display, perfect for personal or photography portfolios
• Demo: https://minimal-gallery-demo.example.com

3. Next Minimalist Portfolio

• SSG: Next.js
• CMS: No CMS
• Features: Clean, content-focused portfolio theme ideal for developers and designers
• Demo: https://minimalist-portfolio-demo.example.com

4. Next Multiple Page Portfolio

• SSG: Next.js
• CMS: No CMS
• Features: Modern multi-page portfolio theme with clear content sections
• Demo: https://multiplepage-portfolio-demo.example.com

✅ All themes include:

• Required front-matter with proper SSG/CMS categorization
• Demo links (placeholder URLs - please update with actual demo URLs)
• English descriptions
• Proper file naming convention (framework-prefixed)

Thank you for reviewing and considering these additions! ��