Add information how to add custom resources to the New Resources menu and an exported field using C#

**Your Godot version:** 4.1 .NET

**Issue description:** Custom resources can be added in the New Resources menu by applying `[GlobalClass]` to it. In addition, to use those classes you can add `[Export(PropertyHint.ResourceType, nameof(YourCustomResource))]` above an exported field to let the editor know which classes to use. This information is currently missing from the docs.

**URL to the documentation page:** https://docs.godotengine.org/en/stable/tutorials/scripting/resources.html#creating-your-own-resources

If you know how to fix the issue you are reporting please
consider opening a pull request. We provide a tutorial on
using git here: https://docs.godotengine.org/en/stable/community/contributing/pr_workflow.html,
writing documentation at https://docs.godotengine.org/en/stable/community/contributing/docs_writing_guidelines.html
and contributing to the class reference here: https://docs.godotengine.org/en/stable/community/contributing/updating_the_class_reference.html