[Docs] GraphicsCompositor - Add XML summary

[VaclavElias]

Describe the update or improvement you would like to see
I would like to add a description summary to GraphicsCompositor, what is it doing and a link to our Stride docs.

Is this related to a specific part of the documentation?
https://doc.stride3d.net/latest/en/manual/graphics/graphics-compositor/index.html

Why is this update needed?
I am going through the basics and architecture of Stride and when we work with GraphicsCompositor, the IntelliSense should provide some basis description.

Additional context
Proposed text, also below in xml.

Proposed changes
I would like to review this addition:

    /// <summary>
    /// The <c>GraphicsCompositor</c> class organizes how scenes are rendered in the Stride engine, providing extensive customization of the rendering pipeline.
    /// </summary>
    /// <remarks>
    /// This class handles the initialization and destruction of the render system, manages the cameras used in the composition, and controls the render stages and features.
    /// It provides entry points for the game compositor, a single view compositor, and a compositor used by the scene editor.
    /// <para>
    /// Key features of the <c>GraphicsCompositor</c> include:
    /// </para>
    /// <list type="bullet">
    ///   <item><description>Using one or multiple cameras</description></item>
    ///   <item><description>Filtering entities</description></item>
    ///   <item><description>Rendering to one or more render textures with different viewports</description></item>
    ///   <item><description>Setting HDR or LDR rendering</description></item>
    ///   <item><description>Applying post effects to a render target</description></item>
    ///   <item><description>Clearing a render target or only the depth buffer</description></item>
    ///   <item><description>Modifying the compositor from scripts</description></item>
    /// </list>
    /// <para>
    /// For more information, see the
    /// <see href="https://doc.stride3d.net/latest/en/manual/graphics/graphics-compositor/index.html">Graphics Compositor</see> documentation.
    /// </para>
    /// </remarks>

[VaclavElias]

Are you ok with this summary? If yes, I will make a PR.