Talk about 9-slice-scaling

This is quite easy to talk about, the concept is simple and works well for UI. Information online is plentiful.

Look up 9-slice scaling for UI, gather online resources, and write a concise explanation for the compendium. The entry should cover the concept, use cases, and examples. Check existing sections for style and integrate the new content appropriately.

索引モデルが issue の本文から書いたものです。