Fix C# examples in documentation

- Fix documentation after C# renames.
- Add missing `partial` in C# class declarations.
- Change `delta` parameter type to `double` in C#.
- Ensure parameters match base declaration.
- Use `$` string interpolation in C#.
- Fix invalid or outdated C# code.
- Changed some examples to follow our style guide more closely.

doc/classes/Tween.xml

@@ -77,13 +77,13 @@
 		    tween = create_tween()
 		[/gdscript]
 		[csharp]
-		private Tween tween;
+		private Tween _tween;
 
 		public void Animate()
 		{
-		    if (tween != null)
-		        tween.Kill(); // Abort the previous animation
-		    tween = CreateTween();
+		    if (_tween != null)
+		        _tween.Kill(); // Abort the previous animation
+		    _tween = CreateTween();
 		}
 		[/csharp]
 		[/codeblocks]