Add light dark image support

[mnkiefer]

• Introduces support for theme-aware (light and dark mode) images in workshop authoring and automation workflows.
• • Updates both authoring guidelines and the automation for explanatory diagrams and UI screenshots, ensuring all new and migrated visuals use paired light/dark SVGs and the <picture> pattern for proper theme switching on GitHub.
• Also enhances automation to detect and migrate existing single-theme images and to audit all image references, including those embedded in HTML.