Add comprehensive theme documentation and improve migration script

- Add THEMES-HOWTO.md: Complete guide for creating and customizing themes
- Remove theme sections from how-it-works.md to avoid duplication
- Update migration script to place all blog posts in single directory
- Streamline documentation structure for better organization

scripts/migrate_qs2_to_qs3.py

@@ -206,10 +206,10 @@ def process_blog_file(file_path, output_dir_base):
     sanitized_title = sanitize_filename(metadata['title'])
     if not metadata['date']:
         # Fallback if date couldn't be parsed, though unlikely for .blog files
-        output_subdir = Path(output_dir_base) / "blog" / "unknown_date"
+        output_subdir = Path(output_dir_base) / "blog"
     else:
-        year, month, day = metadata['date'].split('-')
-        output_subdir = Path(output_dir_base) / "blog" / year / month / day
+        # Put all blog posts in a single directory instead of year/month/day structure
+        output_subdir = Path(output_dir_base) / "blog"
         
     output_subdir.mkdir(parents=True, exist_ok=True)
     output_file_path = output_subdir / f"{sanitized_title}.md"