Clarify and expand documentation for thumbnail / preview functionality #424
Description
The functionality for generating, customizing, and displaying thumbnails/previews (for images, documents, etc.) is crucial but documentation is a bit sparse in some areas. For example, how thumbnail sizes are configured, how caching works, how fallback behavior functions when preview generation fails.
Proposed Work:
Add a section in the docs that describes all available thumbnail / preview settings, with examples (settings.py, template tags).
Document how to override thumbnail templates or styling in Grappelli + FileBrowser.
Describe what happens when a preview cannot be generated (file type not supported, missing libs, etc.).
Possibly provide a small example with automatic cleanup of old thumbnails / cache.
This will help developers using the package to better integrate preview behavior and avoid surprises.