add banner image, tweak repo badges

[jeffpaul]

Description of the Change

This PR adds in the plugin banner image to the readme to give a bit more visual appear to the readme intro to new plugin users/developers. Also updates the badges displayed.

How to test the Change

Preview via the GitHub markdown previewer.

Changelog Entry

n/a

Credits

Props @jeffpaul.

Checklist:

• [x] I agree to follow this project's Code of Conduct.
• [x] I have updated the documentation accordingly.
• [ ] I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• [ ] All new and existing tests pass.