feat(docs): EntityType basics

docs/entities/index.md

@@ -23,7 +23,42 @@ The simplest and most trivial entity does absolutely nothing. Most users will on
 
 ## `EntityType`: The Grounding Principle
 
-TODO
+An `EntityType` is the singleton that defines what an entity is. Multiple entities within the world can be associated with one `EntityType`. The `EntityType` also links an [`EntityRenderer`][entityrenderer] to its corresponding `Entity` class. It also defines default properties that all entities with this type has. The `EntityType` must be [registered].
+
+An `EntityType` can be constructed via `EntityType.Builder#of`. This method takes in two parameters: an `EntityType.EntityFactory` that [constructs a default `Entity` instance][entity], and a [`MobCategory`][mobcategory] indicating the entity type. The `EntityType` can be built via `EntityType.Builder#build` by passing in `null` as the argument.
+
+:::info
+The `String` in the `build` method represents the registry name of the `EntityType` and is used when dealing with data fixers. The reason we pass in `null` is that data fixers go unused in mod development. If a value is passed in, then Vanilla's data fixer will continually throw a warning when loading the game as there is no definition within an applicable schema.
+:::
+
+```java
+// For some entity
+public static class ExampleEntity extends Entity {
+
+    // This is the constructor definition for use in 'EntityType.EntityFactory'
+    public ExampleEntity(EntityType<? extends ExampleEntity> type, Level level) {
+        super(type, level);
+        // ...
+    }
+
+    // ...
+}
+
+// In some class that holds registry objects
+public static final DeferredRegister<EntityType<?>> REGISTRAR = DeferredRegister.create(Registries.ENTITY_TYPE, MOD_ID);
+
+public static final DeferredHolder<EntityType<?>, EntityType<ExampleEntity>> EXAMPLE_ENITTY = REGISTRAR.register(
+    "example_entity",
+    () -> EntityType.Builder.of(
+            ExampleEntity::new // The constructor definition,
+            MobCategory.MISC // Category for entities that do not generally extend 'Mob'
+        ).build(null) // String value goes unused
+);
+```
+
+:::note
+The builder contains many methods that will be further discussed in other sections to help with understanding. An example for applying each to an `EntityType` will be provided there.
+:::
 
 ### Entity Dimensions
 
@@ -69,5 +104,7 @@ TODO
 [entity]: #everything-revolves-around-entity
 [entityrenderer]: ./renderer.md
 [summon]: #summoning-an-entity
+[registered]: ../concepts/registries.md#methods-for-registering
 <!-- Replace with living entity docs -->
 [living]: #
+[mobcategory]: #