[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/index.md

@@ -5,15 +5,14 @@ Backstage is a single-page application composed of a set of plugins.
 Our goal for the plugin ecosystem is that the definition of a plugin is flexible
 enough to allow you to expose pretty much any kind of infrastructure or software
 development tool as a plugin in Backstage. By following strong
-[design guidelines](https://github.com/spotify/backstage/blob/master/docs/design.md)
-we ensure the the overall user experience stays consistent between plugins.
+[design guidelines](../dls/design.md) we ensure the the overall user experience
+stays consistent between plugins.
 
 ![plugin](my-plugin_screenshot.png)
 
 ## Creating a plugin
 
-To create a plugin, follow the steps outlined
-[here](https://github.com/spotify/backstage/blob/master/docs/getting-started/create-a-plugin.md).
+To create a plugin, follow the steps outlined [here](create-a-plugin.md).
 
 ## Suggesting a plugin