GDExtension add documentation on hot reloading feature (C++)

[Zacrain]

Your Godot version:
v4.2.2.stable.official [15073afe3]

Issue description:
With PR 80284 ( godotengine/godot#80284 ) hot reloading was added. I encountered that I wasn't able to use that feature until I looked for and read a bit on the pull request page. For example, it is necessary to add the configuration parameter:

reloadable = true

to the [configuration] section of the corresponding .gdextension file. Yet this seems to be currently undocumented. (Furthermore I have encountered some issues, which I've detailed over at godot-cpp: godotengine/godot-cpp#1502 .)

I suppose the documentation on GDExtension with the C++ example would be a good place to add it. Furthermore, it might be meaningful to add a section detailing the .gdextension file and all possible parameters as I haven't found a separate documentation on that.

Regarding the additional documentation in the GDExtension C++ Example, I'll provide a first draft in a pull request probably in the next couple of days. The latter, whether such an overview manual page might be meaningufl, I'll leave open for discussion.

URL to the documentation page (if already existing):
https://docs.godotengine.org/en/stable/tutorials/scripting/gdextension/gdextension_cpp_example.html

[paddy-exe]

Thanks for raising this issue
Fixed already by #9383