GDScript: Replace abstract keyword with @abstract annotation

Co-authored-by: Danil Alexeev <dalexeev12@yandex.ru>

modules/gdscript/doc_classes/@GDScript.xml

@@ -295,6 +295,27 @@
 		</constant>
 	</constants>
 	<annotations>
+		<annotation name="@abstract">
+			<return type="void" />
+			<description>
+				Marks a class or a method as abstract.
+				An abstract class is a class that cannot be instantiated directly. Instead, it is meant to be inherited by other classes. Attempting to instantiate an abstract class will result in an error.
+				An abstract method is a method that has no implementation. Therefore, a newline or a semicolon is expected after the function header. This defines a contract that inheriting classes must conform to, because the method signature must be compatible when overriding.
+				Inheriting classes must either provide implementations for all abstract methods, or the inheriting class must be marked as abstract. If a class has at least one abstract method (either its own or an unimplemented inherited one), then it must also be marked as abstract. However, the reverse is not true: an abstract class is allowed to have no abstract methods.
+				[codeblock]
+				@abstract class Shape:
+					@abstract func draw()
+
+				class Circle extends Shape:
+					func draw():
+						print("Drawing a circle.")
+
+				class Square extends Shape:
+					func draw():
+						print("Drawing a square.")
+				[/codeblock]
+			</description>
+		</annotation>
 		<annotation name="@export">
 			<return type="void" />
 			<description>