Add Playground badge, fix e2e badge

[jeffpaul]

Description of the Change

Adds a badge linking to a WP Playground instance instead of the release version badge and fixes the e2e badge to the right GH Action link.

How to test the Change

Use GH markdown previewer.

Changelog Entry

n/a

Credits

Props @jeffpaul.

Checklist:

• [x] I agree to follow this project's Code of Conduct.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my change.
• [ ] All new and existing tests pass.