doc: put Render Pipeline Compatibility in the index page (#661)

* put Render Pipeline compatibility on the index page
* remove other render-pipeline requirements, as they are already implicit 
* remove requirements page from the TOC.

Author: sindharta

com.unity.toonshader/Documentation~/System-Requirements.md

@@ -1,5 +1,4 @@
 * [Unity Toon Shader](index.md)
-* [Requirements](System-Requirements.md)
 * [Installation](installation.md)
 * [Getting started](GettingStarted.md)
 * [Material Converter](MaterialConverter.md)

com.unity.toonshader/Documentation~/TableOfContents.md

@@ -25,4 +25,15 @@ Three basic colors—**base color**, **1st shading color**, and **2nd shading co
 
 Besides the three basic colors, the **Unity Toon Shader** provides a wide variety of features such as [Highlight](Highlight.md), [Rim Light](Rimlight.md), [Emission](Emission.md), [Angel Ring](AngelRing.md), [Material Capture (MatCap)](MatCap.md), and some special maps.
 
-To get started with cel-shading, see [Getting Started](GettingStarted.md).
+To get started with cel-shading, see [Getting Started](GettingStarted.md).
+
+## Render Pipeline Compatibility
+The **Unity Toon Shader** works across the **Built-in Render Pipeline**,
+**Universal Render Pipeline (URP)**, and **High Definition Render Pipeline (HDRP)**.
+
+Feature availability can vary slightly between pipelines,
+so review the [feature differences](FeatureModel_en.md) document to confirm
+which features are supported in your target pipeline.
+
+> The use of linear color space is strongly recommended.
+