[docs] add docs for vite-plugin-svelte

documentation/docs/14-configuration.md

@@ -55,6 +55,9 @@ const config = {
 		trailingSlash: 'never',
 		vite: () => ({})
 	},
+
+	// passed to vite-plugin-svelte (https://github.com/sveltejs/vite-plugin-svelte/blob/main/docs/config.md#onwarn)
+	onwarn: () => {},
 
 	// options passed to svelte.preprocess (https://svelte.dev/docs#svelte_preprocess)
 	preprocess: null