docs: Enhance Entity class comment with detailed description. (#2423)

The comment for the `Entity` class has been updated to provide a more detailed description. It now specifies that the class represents a game entity that typically aggregates multiple `EntityComponent` instances. Additionally, it includes references to the Stride documentation for more information about adding and managing entities.

Author: VaclavElias

sources/engine/Stride.Engine/Engine/Entity.cs

@@ -17,7 +17,9 @@
 namespace Stride.Engine
 {
     /// <summary>
-    /// Game entity. It usually aggregates multiple EntityComponent
+    /// Represents a game entity that typically aggregates multiple <see cref="EntityComponent"/> instances.
+    /// For more information about entities, see <see href="https://doc.stride3d.net/latest/en/manual/game-studio/add-entities.html">Adding entities</see>
+    /// and <see href="https://doc.stride3d.net/latest/en/manual/game-studio/manage-entities.html">Managing entities</see>.
     /// </summary>
     //[ContentSerializer(typeof(EntityContentSerializer))]
     //[ContentSerializer(typeof(DataContentSerializer<Entity>))]