message: docs: Add an example code block for debugging new url reader
diff: @@ -242,6 +242,23 @@ HTTP headers.
  When debugging one of the URL Readers, you can straightforward use the
  [`reader` instance created](https://github.com/backstage/backstage/blob/ebbe91dbe79038a61d35cf6ed2d96e0e0d5a15f3/packages/backend/src/index.ts#L57)
  when the backend starts and call one of the methods with your debugging URL.
  +
  +```ts
  +// File: packages/backend/src/index.ts
  +
  +async function main() {
  + // ...
  + const createEnv = makeCreateEnv(config);
  +
  + const testReader = createEnv('test-url-reader').reader;
  + const response = await testReader.readUrl(
  + 'https://github.com/backstage/backstage/blob/master/catalog-info.yaml',
  + );
  + console.log((await response.buffer()).toString());
  + // ...
  +}
  +```
  +
  This will be run every time you restart the backend. Note that after any change
  in the URL Reader code, you need to kill the backend and restart, since the
  `reader` instance is memoized and does not update on hot module reloading. Also,
  