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- Fix invalid or outdated C# code.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/StreamPeer.xml

@@ -223,7 +223,7 @@
 				put_data("Hello world".to_utf8())
 				[/gdscript]
 				[csharp]
-				PutData("Hello World".ToUTF8());
+				PutData("Hello World".ToUtf8());
 				[/csharp]
 				[/codeblocks]
 			</description>