Document `@export_tool_button` and `[ExportToolButton]`

[tetrapod00]

Your Godot version:
4.4
Issue description:
godotengine/godot#96290 adds @export_tool_button for GDscript and godotengine/godot#97894 adds the corresponding [ExportToolButton] for C#. Both need to be documented.

If you are interested in contributing this documentation:

• The original issues provide some example usages.
• Follow the existing format of the two Exported Properties pages. Add a new header, describe the feature, and include a short code example.
• It is okay to only contribute the GDScript or the C# documentation. If you do, just mention that in a comment here.
• We have a tutorial on getting started contributing here: https://docs.godotengine.org/en/stable/contributing/documentation/contributing_to_the_documentation.html

GDScript addresssed by #10338.

URL to the documentation page (if already existing):
GDScript: https://docs.godotengine.org/en/latest/tutorials/scripting/gdscript/gdscript_exports.html
C#: https://docs.godotengine.org/en/latest/tutorials/scripting/c_sharp/c_sharp_exports.html

[52SW]

Hi, I'm trying to contribute to adding document for the @export_tool_button for GDscript and I've submitted a pr.

[tetrapod00]

Thanks! I'll take a look and review soon.

For anyone else: only the C# still needs to be documented.