Document kind, name, plural for entity config

[oandregal]

Part of #55153
Follow-up to #55103

Documents the kind, name and plural properties of an entity.

Read new section.

[github-actions]

Flaky tests detected in 78600ec.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/6455766914
📝 Reported issues:

• [Flaky Test] should insert elements from multiple completers in a single block #47916 in /test/e2e/specs/editor/various/autocomplete-and-mentions.spec.js
• [Flaky Test] should partially select with shift + click (@webkit) #53524 in /test/e2e/specs/editor/various/multi-block-selection.spec.js

[tyxla]

Nice 🚀

[oandregal]

There's a playwright test that is failing now dims background image down by 50% with the average image color when an image is uploaded. I'm not sure how it could have anything to do with a change in this README. I've also re-run a few times to clear it, but it doesn't go away. I don't want to fill the job queue with more tasks than necessary, so I'm going to ship this one.