replace some crefs with direct links (#3878)

com.unity.ml-agents/Runtime/Agent.cs

         internal VectorSensor collectObservationsSensor;
 
         /// <summary>
-        /// Called when the attached <see cref="GameObject"/> becomes enabled and active.
+        /// Called when the attached [GameObject] becomes enabled and active.
+        /// [GameObject]: https://docs.unity3d.com/Manual/GameObjects.html
         /// </summary>
         /// <remarks>
         /// This function initializes the Agent instance, if it hasn't been initialized yet.
         }
 
         /// <summary>
-        /// Called when the attached <see cref="GameObject"/> becomes disabled and inactive.
+        /// Called when the attached [GameObject] becomes disabled and inactive.
+        /// [GameObject]: https://docs.unity3d.com/Manual/GameObjects.html
         /// </summary>
         /// <remarks>
         /// Always call the base Agent class version of this function if you implement `OnDisable()`
 
         /// <summary>
         /// Set up the list of ISensors on the Agent. By default, this will select any
-        /// SensorBase's attached to the Agent.
+        /// SensorComponent's attached to the Agent.
         /// </summary>
         internal void InitializeSensors()
         {

com.unity.ml-agents/Runtime/Sensors/ISensor.cs

 
     /// <summary>
     /// Sensor interface for generating observations.
-    /// For custom implementations, it is recommended to <see cref="SensorBase"/> instead.
     /// </summary>
     public interface ISensor
     {
 
         /// <summary>
         /// Write the observation data directly to the <see cref="ObservationWriter"/>.
-        /// This is considered an advanced interface; for a simpler approach, use
-        /// <see cref="SensorBase"/> and override <see cref="SensorBase.WriteObservation"/> instead.
         /// Note that this (and  <see cref="GetCompressedObservation"/>) may
         /// be called multiple times per agent step, so should not mutate any internal state.
         /// </summary>

com.unity.ml-agents/Runtime/Sensors/RenderTextureSensor.cs

 namespace Unity.MLAgents.Sensors
 {
     /// <summary>
-    /// Sensor class that wraps a <see cref="RenderTexture"/> instance.
+    /// Sensor class that wraps a [RenderTexture](https://docs.unity3d.com/ScriptReference/RenderTexture.html) instance.
     /// </summary>
     public class RenderTextureSensor : ISensor
     {
         /// <summary>
         /// Initializes the sensor.
         /// </summary>
-        /// <param name="renderTexture">The <see cref="RenderTexture"/> instance to wrap.</param>
+        /// <param name="renderTexture">The [RenderTexture](https://docs.unity3d.com/ScriptReference/RenderTexture.html)
+        /// instance to wrap.</param>
+        /// [GameObject]: https://docs.unity3d.com/Manual/GameObjects.html
         public RenderTextureSensor(
             RenderTexture renderTexture, bool grayscale, string name, SensorCompressionType compressionType)
         {
         public void Update() {}
 
         /// <inheritdoc/>
-        public void Reset() { }
+        public void Reset() {}
 
         /// <inheritdoc/>
         public SensorCompressionType GetCompressionType()

com.unity.ml-agents/Runtime/Sensors/RenderTextureSensorComponent.cs

         RenderTextureSensor m_Sensor;
 
         /// <summary>
-        /// The <see cref="UnityEngine.RenderTexture"/> instance that the associated
-        /// <see cref="RenderTextureSensor"/> wraps.
+        /// The [RenderTexture](https://docs.unity3d.com/ScriptReference/RenderTexture.html) instance
+        /// that the associated <see cref="RenderTextureSensor"/> wraps.
-        /// Stores the <see cref="UnityEngine.RenderTexture"/> associated with this sensor.
+        /// Stores the [RenderTexture](https://docs.unity3d.com/ScriptReference/RenderTexture.html)
+        /// associated with this sensor.
         /// </summary>
         public RenderTexture RenderTexture
         {