docs: Update build-caching.md to mention that build outputs are also …

…in public (#20869)

docs/docs/build-caching.md

@@ -9,7 +9,7 @@ Caching is already used by Gatsby and plugins for example:
 - any nodes created by source/transformer plugins are cached
 - `gatsby-plugin-sharp` caches built thumbnails
 
-Data is stored in the `.cache` directory relative to your project root.
+Build outputs are stored in the `.cache` and `public` directories relative to your project root.
 
 ## The cache API