Update/readme

[jeffpaul]

Description of the Change

This PR reorganizes the README.md file and merges in the "WIP" documentation sections into the normal flow of the readme. Documentation is, unfortunately, always a work in progress so it seemed better to pull that content in as-is and we can continue to iterate from there.

This PR also makes use of the nametag image from GatherPress.org and turns it into the WP.org plugin icon image and includes the various versions for publishing on WP.org.

This PR also pulls in the demo video from earlier this month on GatherPress.org in hopes of rendering that in the README.md to give folks a good demo/preview in GitHub as well.

Still needed is:

• Update the description of the GatherPress repo from the No description, website, or topics provided. placeholder to GatherPress, powering our community's event management needs. or something like that.
• Generate a plugin header image and add to the repo (.wordpress-org folder so they'll get deployed to WP.org on the next release published there)
• Generate some plugin screenshots and add those to the repo and reference in the readme files

Closes #50, #406.

How to test the Change

Use the GitHub markdown previewer to see the file changes.

Changelog Entry

Added - Plugin icon files.
Changed - Updated documentation within readme files.

Credits

Props @jeffpaul.

Checklist:

• 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.