[Docs] Fix relative paths to code (#1662)

* [Docs] Fix relative paths to code

* Update create-a-plugin.md

* Link to existing plugins

* Update README.md

* Update README.md

* Review edits

docs/plugins/create-a-plugin.md

@@ -24,7 +24,7 @@ for your new plugin directly by navigating to
 `http://localhost:3000/my-plugin`._
 
 <p align='center'>
-    <img src='https://github.com/spotify/backstage/raw/master/docs/getting-started/my-plugin_screenshot.png' width='600' alt='my plugin'>
+    <img src='https://github.com/spotify/backstage/raw/master/docs/plugins/my-plugin_screenshot.png' width='600' alt='my plugin'>
 </p>
 
 You can also serve the plugin in isolation by running `yarn start` in the plugin