docs: update vite documentation for migrating the custom webpack config

[jleifeld]

We have already quite a bit of documentation for the Vite migration: https://developer.shopware.com/docs/guides/plugins/plugins/administration/system-updates/vite.html

But one thing missing is an example for plugin developer which are using a custom Webpack configuration. They need to convert to a Vite configuration. In many cases this is very individual but the example should just cover the main cases:

• Move the config from the administration folder to administration/src
• Rename the config to the vite default standard
• Implement a alias as an example

[bojanrajh]

Suggested change

	@example: 'src/example',
	`@example`: 'src/example',