docs: clarify that you can build SPAs with adapter-static

closes #8396

Author: dummdidumm

.changeset/pink-bulldogs-remain.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-static': patch
+---
+
+docs: clarify that you can build SPAs with adapter-static

packages/adapter-static/README.md

@@ -1,6 +1,6 @@
 # @sveltejs/adapter-static
 
-[Adapter](https://kit.svelte.dev/docs/adapters) for SvelteKit apps that prerenders your entire site as a collection of static files. If you'd like to prerender only some pages, you will need to use a different adapter together with [the `prerender` option](https://kit.svelte.dev/docs/page-options#prerender).
+[Adapter](https://kit.svelte.dev/docs/adapters) for SvelteKit apps that prerenders your entire site as a collection of static files. It's also possible to create an SPA with it by specifying a fallback page which renders an empty shell. If you'd like to prerender only some pages and not create an SPA for those left out, you will need to use a different adapter together with [the `prerender` option](https://kit.svelte.dev/docs/page-options#prerender).
 
 ## Usage