Mention runtimeChunk: single for multi-endpoint dev server

This section of the guide introduces the concept of a dev server for the first time, but has not introduced the concept that multiple endpoints on a single html file served by the webserver is problematic without `optimization.runtimeChunk: 'single'`.

This commit attempts to give new developers a softer landing, and references the Code Splitting section to more deeply understand as well as link to the same pitfall external resource as the Code Splitting section does.

See webpack/webpack#14873 (comment) for rationale

Author: f3ndot

src/content/guides/development.mdx

@@ -225,11 +225,16 @@ Change your configuration file to tell the dev server where to look for files:
      path: path.resolve(__dirname, 'dist'),
      clean: true,
    },
++  optimization: {
++    runtimeChunk: 'single',
++  },
  };
 ```
 
 This tells `webpack-dev-server` to serve the files from the `dist` directory on `localhost:8080`.
 
+The `optimization.runtimeChunk: 'single'` was added because in this example we have more than one entrypoint on a single HTML page. Without this, we could get into trouble described [here](https://bundlers.tooling.report/code-splitting/multi-entry/). Read the [Code Splitting](/guides/code-splitting/) section of the guide for more details.
+
 T> `webpack-dev-server` serves bundled files from the directory defined in [`output.path`](/configuration/output/#outputpath), i.e., files will be available under `http://[devServer.host]:[devServer.port]/[output.publicPath]/[output.filename]`.
 
 W> webpack-dev-server doesn't write any output files after compiling. Instead, it keeps bundle files in memory and serves them as if they were real files mounted at the server's root path. If your page expects to find the bundle files on a different path, you can change this with the [`devMiddleware.publicPath`](/configuration/dev-server/#devserverdevmiddleware) option in the dev server's configuration.