Add documentation for the GCI95 rule

This is for green-code-initiative/creedengo-csharp#92

Author: StanleySweet

src/main/rules/GCI95/GCI95.json

@@ -0,0 +1,16 @@
+{
+  "title": "Use 'is' operator instead of 'as' operator and null check",
+  "type": "CODE_SMELL",
+  "status": "ready",
+  "remediation": {
+    "func": "Constant/Issue",
+    "constantCost": "1min"
+  },
+  "tags": [
+    "roslynator",
+    "readability",
+    "simplification",
+    "performance"
+  ],
+  "defaultSeverity": "Minor"
+}

src/main/rules/GCI95/csharp/GCI95.asciidoc

@@ -0,0 +1,45 @@
+Using the `as` operator followed by a null check is more verbose and can be simplified using the `is` operator with pattern matching.
+
+When using the `is` pattern, the type check and cast happen in one concise statement, improving readability and reducing boilerplate.
+
+Therefore, using `is` with pattern matching will simplify code and improve maintainability.
+
+== Non compliant Code Example
+
+[source,csharp]
+----
+var myObj = obj as MyType;
+if (myObj != null)
+{
+    myObj.DoSomething();
+}
+----
+
+== Compliant Code Example
+
+[source,csharp]
+----
+if (obj is MyType myObj)
+{
+    myObj.DoSomething();
+}
+----
+
+[source,csharp]
+----
+var anotherObj = maybeObj as AnotherType;
+if (anotherObj != null)
+{
+    Console.WriteLine(anotherObj.Value);
+}
+----
+
+== Compliant Code Example
+
+[source,csharp]
+----
+if (maybeObj is AnotherType anotherObj)
+{
+    Console.WriteLine(anotherObj.Value);
+}
+----