document how to override the default port

Port 3000 is a common port in the development environment.

This documents how you can start backstage with a different port to the default.

Moved section for development environment to own page within docs in order to keep README small.

docs/getting-started/README.md

@@ -2,6 +2,7 @@
 
 Here is a collection of tutorials that will guide you through setting up and extending an instance of Backstage with your own plugins.
 
+- [Development Environment](development-environment.md)
 - [Create a Backstage plugin](create-a-plugin.md)
 - [Structure of a plugin](structure-of-a-plugin.md)
 - Using Backstage components (TODO)