docs: split and expand backend docs (#3350)

website/content/docs/test-backend.md

@@ -0,0 +1,16 @@
+---
+title: Test
+weight: 30
+group: backends
+---
+
+You can use the `test-repo` backend to try out Netlify CMS without connecting to a Git repo. With this backend, you can write and publish content normally, but any changes will disappear when you reload the page. This backend powers the Netlify CMS [demo site](https://cms-demo.netlify.com/).
+
+**Note:** The `test-repo` backend can't access your local file system, nor does it connect to a Git repo, thus you won't see any existing files while using it.
+
+To enable this backend, add the following lines to your Netlify CMS `config.yml` file:
+
+```yaml
+backend:
+  name: test-repo
+```