Entity Attributes: Move Code to ExampleMod

[its-miroma]

Now that #267 is merged 🎉, including some Entity-related code, the code blocks we had embedded in the Entity Attributes file should be tracked in the reference mod, so they don't break with updates.

fabric-docs/develop/entities/attributes.md

Line 61 in 1dc5332

<!-- TODO: move to the reference mod -->