Add note to docs about svelte-transition

[CaptainCodeman]

Make it clear that svelte-transition is a separate package that needs to be installed if you want to use it in your own component (as shown in the examples).

It's not a dependency of svelte-headlessui itself, although it may be worth considering adding it to this package (as it will likely be used more often than not).

[colinbate]

When I installed svelte-headlessui, the svelte-transition came with it. It is listed as a dependency in here: https://github.com/CaptainCodeman/svelte-headlessui/blob/master/packages/lib/package.json

That said, I agree that it doesn't need to be a dependency. I didn't use it myself because it gave me issues with SSR in SvelteKit.