Add a page for GDExtension's extension_api.json files

[Ivorforce]

Motivated by David's comment: Currently, the GDExtension section links to godot-cpp instead of the other way around. This PR changes this.

While trying to figure out the structure, I found that it makes sense to link to a page about extension_interface.json in various places.
We currently do not have information about the extension_api.json files in the docs.

... well, and we still don't, which is why this change is in draft :)
cc @dsnopek, would you be available to write the documentation for this file?

[Bromeon]

How detailed should a gdextension_api.json doc be?

I'm questioning the value of documenting every possible key-value pair, as 90% are self-explanatory (even more so in case there's a schema denoting possible values)...

Maybe rather focus on high-level structure and some concepts that may not be obvious? 🤔

[Ivorforce]

How detailed should a gdextension_api.json doc be?

I'm questioning the value of documenting every possible key-value pair, as 90% are self-explanatory (even more so in case there's a schema denoting possible values)...

Maybe rather focus on high-level structure and some concepts that may not be obvious? 🤔

Certainly just as detailed as is useful! I think if we link the schema and describe the general structure that might already be very useful.

[dsnopek]

I think if we link the schema and describe the general structure that might already be very useful.

We don't have a schema for extension_api.json yet (only gdextension_interface.json), so maybe that's the first course of action?