Add example of GDExtension written pure C

[dsnopek]

This salvages @vnen's PR #8751

I applied almost all the suggestions there (I disagreed with a small handful of them :-)).

I think vnen had a repo with the full project somewhere which I need to dig up, so I can update it for the few code changes here and test that everything actually works as written.

[tetrapod00]

Reviewed for grammar, style, and formatting. Generally looks good, overall very clear. I made a bunch of style nitpick suggestions.

[tetrapod00]

FYI in the docs repo we currently only use milestones for PRs that only apply to the latest minor version, in this case 4.4. Everything else gets no milestone

(Maybe we should use them more. But we don't 🤷 )

[mhilbrunner]

LGTM, read it twice for proofreading, and approving the concept of this tutorial. I didn't have time to actually test any of it, but I'll trust vnen and dsnopek on the technical accuracy :)

[dsnopek]

Per my original note in the description:

I think vnen had a repo with the full project somewhere which I need to dig up, so I can update it for the few code changes here and test that everything actually works as written.

I've cloned vnen's repo and tested all the code updates that were made to the tutorial - you can find my updated version here:

https://github.com/dsnopek/gdextension-c-example/tree/drs

So, the current code in the tutorial is all tested as working now :-)

We should probably move the complete code to somewhere official, although, I'm not exactly sure where?

[mhilbrunner]

We should probably move the complete code to somewhere official, although, I'm not exactly sure where?

We have (had) https://github.com/godotengine/gdnative-demos once upon a time. It may make sense to createa gdextension-demos repo. Should probably be discussed with Akien.

[mhilbrunner]

Thank you! Also pinged Akien about the repo in #gdextension on RC.