books: Add several books

Add Polish books

• GIMP Naucz się tworzenia grafiki i obróbki zdjęć
• GIMP obróbka grafiki i zdjęć 50 trików
• GIMP 24 lekcje
• GIMP Kurs obsługi w przykładach
• GIMP grafika w Linuksie
• Korekta fotografii w programie GIMP

Add English books

• Color Interpolation Techniques: GNU Image Manipulation Package (GIMP) Perspective
• Instant GIMP Starter
• GIMP for Beginners: First 12 Skills
• Learn GIMP: Top 10 Artistic Techniques
• Learn GIMP: Top 10 New Techniques
• Learn GIMP: Top 10 Professional Techniques
• Digital Image Optimization Concepts and Techniques
• How to Create Professional Book Covers
• GIMP for Beginners: Book Covers and Free Graphic Design
• How to Make an E-Book Cover with GIMP
• Creating Book Covers with the GIMP for Self-publishing Beginners
• Using GIMP
• Quick free world maps using GIMP filters and Donjon fractal maps

Add a JSON DB

• Convert index.md to JSON format.