(docs) Memory leak in XState recipe

[loganvolkers]

The XState example recipe contains a memory leak.

Since start() is called inside of the molecule, but there is no call to stop(), it creates state that can never be cleaned up.

This was originally reported in #35, but has been split out as it's own issue to make sure we aren't recommending a bad recipe.

This isn’t an issue with jotai or nanostores since they already have a cleanup mechanism, but when working with xstate there's no way to stop the machine, which leads to a memory leak.

Demo of the problem: https://codesandbox.io/s/molecule-cleanup-mvt4cg?file=/App.tsx

[loganvolkers]

Fixed in #44