Add a link to the GDExtension development in the expected location

**Godot 4.6.3:**

I can't find any links or pages for the GDExtension C++ tutorial. The last time I used one was back in Godot 4.4, and I found it without any problems.

I don't remember where I found this, but as a user, I'd like to see it on the [Plugins](https://docs.godotengine.org/en/stable/tutorials/plugins/index.html) page. This section is most appropriate, since GDExtension is also a plugin. Perhaps there should be a separate section on [contributing.godotengine.org](https://contributing.godotengine.org/en/latest/organization/how_to_contribute.html) (Although this is a less suitable place).

**[Plugins](https://docs.godotengine.org/en/stable/tutorials/plugins/index.html)**

Start with the Plugins documentation page linked in the issue and check the referenced contributing page as an alternative location. Locate the GDExtension C++ tutorial or development material, then add an appropriate link in the expected documentation section and verify that it renders and reaches the intended page.

Written by the indexing model from the issue text.