Add button examples for hover and focus states of variants

[owenatgov]

Adds examples to button for:

• forced hover state of all variants
• disabled state of all variants
• forced focus state, only of the default button since it's the same for all variants

Part of #6257 although will not resolve it

We previously removed a lot of these examples in #6091 however the design system designers expressed that it'd be useful to see these examples at a glance, in particular the different colours.

This could be remedied more completely by some sort of states API as detailed in #6257 (comment), but this PR does the job for now

[github-actions]

📋 Stats

No changes to any distributed file sizes!

---

Action run for e8f5f6a

[romaricpascal]

Looking good for the added states, should the :active state be on there as well?

Wondering also if we should have the focus styles on there as well to see if there's any styles for the variant leaking in the focus style when they shouldn't.

Same could be said for having combinations (:hover + :active, :focus + :hover, :focus + :hover + :active) but that starts to be a lot 😓

[owenatgov]

@romaricpascal I'm finally coming back to this. I've updated the PR to include:

• active states per variant
• hover states per variant as opposed to just the one

I tried doing at least one combination to get the yellow outline when you have focus combined with another state but I couldn't get it to work. I wonder if there might be something wrong with our review app pseudo classes, but I'd like to propose this be out of scope of this PR.