doc: move style guide to findable location
Move the style guide to doc/README.md so people might find it. The current location is easily overlooked, as is evidenced by https://github.com/nodejs/node/pull/41025
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Move the style guide to doc/README.md so people might find it. The current location is easily overlooked, as is evidenced by https://github.com/nodejs/node/pull/41025