Documentation improvements: Missing Plugin registration part.

[knowacki23]

Hey,
I've found one missing part in the documentation on how to setup a plugin.
In order to get the data from app-config.yaml file, the gitpod.baseUrl variable to be precise, you need to register the plugin first.

To register a frontend-plugin you need to add it to your packages/app/src/App.tsx file like so:

+ import { GitpodPlugin } from "@gitpod/backstage-plugin-gitpod";

const app = createApp({
  apis,
+  plugins: [GitpodPlugin],
  components: {
    SignInPage: (...),
  },
  bindRoutes({ bind }) {
    (...)
  },
(...)
});

Then the Gitpod plugin will be able to read the custom Gitpod URL string from app-config.yaml.