Add aria-disabled state

[simurai]

This is an alternative to #983 and in the same spirit as #1006.

But instead of replacing the .disabled class, it only adds [aria-disabled=true]. Making it easier to introduce without the need to refactor dotcom's many places where currently the .disabled class and disabled attribute is being used.

• Add [aria-disabled=true]
• Update docs

Note: The disabled attribute and .disabled class can still be used for styling, it's just not encouraged and therefore not documented.

---

Ref. #791
Superseeds and closes #983

[vercel]

This pull request is being automatically deployed with ZEIT Now (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://zeit.co/primer/primer-css/b9ppdi12g
✅ Preview: https://primer-css-git-aria-disabled.primer.now.sh