[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/getting-started/development-environment.md

@@ -50,7 +50,8 @@ system resources and slow things down.
 
 ## Package Scripts
 
-There are many commands to be found in the root [package.json](package.json),
+There are many commands to be found in the root
+[package.json](https://github.com/spotify/backstage/blob/master/package.json),
 here are some useful ones:
 
 ```python
@@ -80,7 +81,9 @@ yarn diff # Make sure all plugins are up to date with the latest plugin template
 yarn create-plugin # Create a new plugin
 ```
 
-> See [package.json](/package.json) for other yarn commands/options.
+> See
+> [package.json](https://github.com/spotify/backstage/blob/master/package.json)
+> for other yarn commands/options.
 
 [Next Step - Create a Backstage plugin](../plugins/create-a-plugin.md)