Warn about non-tool plugin scripts in C#

[raulsntos]

• Add warning about C# plugins not reloaded when they are not tools
• Add [Tool] attribute to plugin scripts in examples so they are not lost on reload (see https://github.com/godotengine/godot/issues/64381)
• This only applies to 3.x since https://github.com/godotengine/godot/pull/65266 is risky and won't be backported.

Not sure if using a list inside a warning is ok but I think it looks kind of ugly and I was getting an error but it seemed to work anyway.

tutorials/plugins/editor/making_plugins.rst:79: ERROR: Unexpected indentation.