[URP] SingleCameraRequest : Add missing RenderPipelineManager Begin/End Camera and Context callbacks

SingleCameraRequest on URP are rendering a single camera, it's purpose is rendering a camera without the full list of stack cameras. This avoid some overhead.

As we are rendering 1 single camera, we are missing:

```
RenderPipelineManager.beginContextRendering
RenderPipelineManager.endContextRendering
RenderPipelineManager.beginCameraRendering
RenderPipelineManager.endCameraRendering
```
Jira Issue: https://jira.unity3d.com/browse/UUM-69268

I've added a new docs page in URP with an exaple of how to use SubmitRenderRequest and how to subscribe to RenderPipelineManager callbacks in each case.

Author: alex-vazquez-unity3d

Packages/com.unity.render-pipelines.core/Documentation~/User-Render-Requests.md

@@ -89,3 +89,7 @@ public class StandardRenderRequest : MonoBehaviour
 }
 ```
 
+## Other useful information
+
+- On [Universal Render Pipeline (URP)](https://docs.unity3d.com/Packages/com.unity.render-pipelines.universal@latest/User-Render-Requests.html).
+

Packages/com.unity.render-pipelines.universal/Documentation~/TableOfContents.md

@@ -100,6 +100,7 @@
     * [Render a camera's output to a Render Texture](rendering-to-a-render-texture.md)
   * [Customize a camera](universal-additional-camera-data.md)
   * [Camera component properties](camera-component-reference.md)
+  * [Render Requests](User-Render-Requests.md)
 * [Post-processing](integration-with-post-processing.md)
   * [How to configure](integration-with-post-processing.md#post-proc-how-to)
   * [HDR Output](post-processing/hdr-output.md)

Packages/com.unity.render-pipelines.universal/Documentation~/User-Render-Requests.md

@@ -0,0 +1,88 @@
+# Render Requests
+
+For a general documentation see the [Core Package](https://docs.unity3d.com/Packages/com.unity.render-pipelines.core@latest/User-Render-Requests.html) about Render Requests.
+
+## Use UniversalRenderPipeline.SingleCameraRequest
+
+`UniversalRenderPipeline.SingleCameraRequest` renders a single camera, without taking into account the full stack of cameras.
+
+You can still hook into callbacks from [RenderPipelineManager](https://docs.unity3d.com/ScriptReference/Rendering.RenderPipelineManager.html).
+
+The following code sample shows that you can hook into [RenderPipelineManager.endContextRendering](https://docs.unity3d.com/ScriptReference/Rendering.RenderPipelineManager-endContextRendering.html) `UniversalRenderPipeline.SingleCameraRequest`
+
+To try out this example:
+
+- Attach the script to a **GameObject** in the **Scene**.
+- Configure the **cams** and **rts**.
+- Set **useSingleCameraRequestValues** to true or false depending on which type of render request you want to use.
+- Select **Enter Play Mode**.
+- See the **Console** Log.
+
+```
+using System.Collections;
+using System.Collections.Generic;
+using System.Text;
+using UnityEngine;
+using UnityEngine.Rendering;
+using UnityEngine.Rendering.Universal;
+
+public class SingleCameraRenderRequestExample : MonoBehaviour
+{
+    public Camera[] cams;
+    public RenderTexture[] rts;
+
+    void Start()
+    {
+        if (cams == null || cams.Length == 0 || rts == null || cams.Length != rts.Length)
+        {
+            Debug.LogError("Invalid setup");
+            return;
+        }
+
+        StartCoroutine(RenderSingleRequestNextFrame());
+        RenderPipelineManager.endContextRendering += OnEndContextRendering;
+    }
+
+    void OnEndContextRendering(ScriptableRenderContext context, List<Camera> cameras)
+    {
+        var stb = new StringBuilder($"Cameras Count from EndContextRendering: <b> {cameras.Count}</b>.");
+        foreach (var cam in cameras)
+        {
+            stb.AppendLine($"- {cam.name}");
+        }
+        Debug.Log(stb.ToString());
+    }
+
+    void OnDestroy()
+    {
+        RenderPipelineManager.endContextRendering -= OnEndContextRendering;
+    }
+
+    IEnumerator RenderSingleRequestNextFrame()
+    {
+        yield return new WaitForEndOfFrame();
+
+        SendSingleRenderRequests();
+
+        yield return new WaitForEndOfFrame();
+
+        StartCoroutine(RenderSingleRequestNextFrame());
+    }
+
+    void SendSingleRenderRequests()
+    {
+        for (int i = 0; i < cams.Length; i++)
+        {
+            UniversalRenderPipeline.SingleCameraRequest request =
+                new UniversalRenderPipeline.SingleCameraRequest();
+
+            // Check if the request is supported by the active render pipeline
+            if (RenderPipeline.SupportsRenderRequest(cams[i], request))
+            {
+                request.destination = rts[i];
+                RenderPipeline.SubmitRenderRequest(cams[i], request);
+            }
+        }
+    }
+}
+```