[chore] update readme and docs demos

[neilhsmith]

Closes #955.

Changes proposed:

• Update the readme and doc's demos. Updating to a modern version of react-modal fixes the accessibility issues noted in the issue automatically.
• Remove the setAppElement example. A warning is now logged when the app element is not set so removed this one and instead setAppElement in all other demos.
• Add a new styled-components example.

The demos themselves have remained largely unchanged apart from converting to functional components. They are still styled simply and do not set any extra props. All demos are stored in this CodePen collection.

Upgrade Path (for changed or removed APIs):

Acceptance Checklist:

• [x] The commit message follows the guidelines in CONTRIBUTING.md.
• [x] Documentation (README.md) and examples have been updated as needed.
• [x] If this is a code change, a spec testing the functionality has been added.
• [x] If the commit message has [changed] or [removed], there is an upgrade path above.

[diasbruno]

Remove the setAppElement example. A warning is now logged when the app element is not set so removed this one and instead setAppElement in all other demos.

This needs to come back. setAppElement has been a hard feature to explain to users.