Add docs for disableNitroPlugin

[letoast]

https://github.com/enkot/nuxt-open-fetch/blob/main/src/module.ts#L32

It would be also nice to document the ${client} = useNitroApp() server functionality in general.

And also extending the type of useNitroApp(). Currently I'm solving this locally by adding a:

// open-api-fetch.d.ts
declare module 'nitropack' {
	interface NitroApp {
		$pets: OpenFetchClient<PetsPaths>
	}
}

export {}

[enkot]

@letoast Updated the docs - https://nuxt-open-fetch.vercel.app/utils/dollarclient. Types should work out of the box, could you please recheck?