Show how to generate a unique ID to use with input labels

[jenweber]

trafficstars

After https://github.com/ember-learn/guides-source/pull/1365 is merged, I think we should show how to use guidFor to generate the ID. There is a good example in the Ember Octane Cheatsheet https://ember-learn.github.io/ember-octane-vs-classic-cheat-sheet/

[rwjblue]

https://github.com/emberjs/rfcs/issues/612 is related (and spit-balling solving this in a nice way).

[ijlee2]

As an update, #1365 has been merged.

[bertdeblock]

We can now use the built-in unique-id helper for this.